This is an appeal from the denial of At-well Junior Conner’s petition for the writ of habeas corpus by the United States District Court for the Northern District of Iowa.1 Conner v. Director of Division of Adult Corrections, No. C 85-0140, slip op. (N.D. Iowa 1987) (unpublished). Conner argues that the State of Iowa holds him “in custody in violation of the Constitution or laws * * * of the United States.” 28 U.S.C. § 2254. The primary issue before us concerns the Iowa felony-murder statute as it relates to an unbroken chain of multiple, contemporaneous or immediate crimes. For the reasons set out below, we affirm the district court’s denial of Conner’s petition.
1. BACKGROUND2
During the evening of March 8, 1974, Conner, along with two friends, George Nowlin and Steve Martin, discussed the possibility of committing a robbery. The next day, Nowlin and Conner armed themselves with two shotguns and, at approximately 10:30 p.m., began to search for victims.
Close to midnight, while driving in Now-lin’s car along a highway in Cedar Rapids, Nowlin and Conner saw a boy and a girl (Michael Servey and Maureen Ann Connolly) walking by the roadside. Their car had run out of gas nearby. Nowlin decided to rob the young couple. He ordered Conner into the backseat of the car and handed one of the shotguns to him. Then Nowlin, pointing the other shotgun at the young couple, ordered Maureen into the front passenger seat of his car and Michael into the backseat with Conner, who held his shotgun at Michael.
Nowlin then demanded Michael’s money. Michael gave it to Conner, who then gave it to Nowlin. Nowlin drove to Mount Vernon, where he stopped on a rural gravel road at approximately 1:00 a.m. He ordered Maureen to get out of the car, and raped her near the car while Conner stayed in the backseat with Michael. Maureen attempted to run away, but Nowlin killed her with two shotgun blasts. Leaving her body in a ditch, Nowlin returned to the car and ordered Conner to drive. When they reached Palisades Park in Linn County, Nowlin got out of the car with Michael. He struck Michael with the shotgun and then shot him dead.
Nowlin and Conner left Michael’s body there and returned to Cedar Rapids be*1386tween 2:00 a.m. and 3:00 a.m. From the beginning of the robbery to the second killing, under three hours had elapsed.
Conner and Martin later helped Nowlin to conceal some of the evidence of the crimes. Eight days after the killing, Martin went to the police and implicated Conner and Nowlin. Conner later confessed to police in the Cedar Rapids stationhouse that he had participated in the crimes.
Conner was convicted of first degree murder under Iowa’s felony-murder statute3 and given a life sentence. The Supreme Court of Iowa affirmed the conviction on direct appeal, holding, inter alia, that the trial court had ruled correctly on the admissibility of evidence offered by the prosecution and the propriety of several jury instructions requested by Conner. Conner I, 241 N.W.2d 447 (Iowa 1976). Conner then filed a habeas petition, which the United States District Court dismissed.
The Eighth Circuit Court of Appeals, in Conner v. Auger, 595 F.2d 407 (1979), affirmed the dismissal, but it vacated the district court’s ruling on Conner’s unex-hausted arguments that the trial court’s application of Iowa’s felony-murder statute to his case was unconstitutional. The Iowa district court and Supreme Court, Conner II, 362 N.W.2d 449, 455 (Iowa 1985), then denied relief in defendant’s post-conviction proceedings, giving rise to the instant ha-beas petition.
II. DISCUSSION
A. Exhaustion
We begin by noting that Conner has now “exhausted the remedies available in the courts of the State.” 28 U.S.C. § 2254(b). The last time Conner was before this court, we held that of the six issues he presented, four had been properly dismissed by the district court, while two remained unex-hausted since they had not been fully presented in the Iowa State court system. Therefore, we vacated the section of the district court’s dismissal of Conner’s petition dealing with the two unexhausted claims so the claims could first be presented in state court.
In the two unexhausted claims, Conner asserted that (1) the trial court erroneously allowed malice aforethought (an element of first degree murder) to be imputed to him and (2) the Iowa felony-murder statute was unconstitutionally applied to his case because the trial court’s jury instructions did not present his theory of defense.4 In Conner II, the Supreme Court of Iowa, in a post-conviction relief proceeding, upheld the Iowa district court’s dismissal of both claims, holding that under § 690.2, (1) Conner was not entitled to an instruction that the State must prove a causal relationship between the underlying felony of robbery and murder and (2) Conner could be convicted of first degree murder based on Iowa’s felony-murder statute without being shown to have personally participated in the killing with malice aforethought. Id. at 453, 455.
However, although Conner’s state claims are now exhausted, the district court is correct to point out that “[t]his court lacks jurisdiction under 28 U.S.C. § 2254 to hear complaints regarding the propriety of state postconviction relief proceedings.” Slip op. at 7. This statement is well-supported by Eighth Circuit precedent. In 1981, the court explained that:
Errors or defects in the state post-conviction proceeding do not, ipso facto, render a prisoner’s detention unlawful or raise constitutional questions cognizable in ha-beas corpus proceedings. Habeas corpus in the federal courts does not serve as an additional appeal from state court convictions. Even where there may be some error in state post-conviction proceedings, this would not entitle appellant to federal habeas corpus relief since appellant’s claim here represents an attack on *1387a proceeding collateral to detention of appellant and not on the detention itself
Williams v. State of Missouri, 640 F.2d 140, 143 (8th Cir.), cert. denied, 451 U.S. 990, 101 S.Ct. 2328, 68 L.Ed.2d 849 (1981).5 Having established that Conner has now exhausted his state claims, we proceed to the merits of Conner’s claims.
B. The Iowa Felony-Murder Statute
First, Conner argues that the trial court erred (thereby depriving him of fourteenth amendment due process) by failing to prove beyond a reasonable doubt that Conner had the requisite mental state to kill the victim or that he participated in the actual homicide. We believe this argument is based on a fundamental misinterpretation of the Iowa felony-murder statute.
Section 690.2 of the Iowa Criminal Code, in pertinent part, reads: “All murder * * * which is committed in the perpetration or attempt to perpetrate any arson, rape, robbery, mayhem or burglary is murder in the first degree * * As the Iowa Supreme Court in Conner I notes, section 690.2 is a distinctive statute. “Unlike the common law felony-murder rule, and statutes in most other jurisdictions, § 690.2 does not make all killings in perpetration of the designated felonies murder. It makes murder in perpetration of such felonies first-degree murder.” 241 N.W.2d at 463.
In light of this unique statute and the Iowa cases that have applied it, we conclude that Conner’s conviction under the statute is constitutional. In Iowa felony-murder cases, it is unnecessary to prove that the defendant participated in the actual homicide. In fact, State v. Phams, 342 N.W.2d 792, 795 (Iowa 1983) (citing State v. Aswegan, 331 N.W.2d 93, 98 (Iowa 1983)), emphasizes that “it is only required that the state prove [that] defendant was participating in the underlying felony.” This is the controlling principle even if the underlying felony was uncompleted or unsuccessful. See Iowa Code § 702.13 (person “participates” in offense whether it is successful or unsuccessful). Moreover, the Supreme Court of Iowa, in Conner II, emphasizes that “ ‘the overwhelming weight of authority is to the effect that if the homicide is committed in what is referred to as the res gestae of the [felony], that is, in connection with it, the killing constitutes murder.’ ” Id. at 463 (citing United States v. Naples, 192 F.Supp. 23 (D.D.C.1961), reversed on other grounds, 113 U.S.App.D.C. 281, 307 F.2d 618 (1962).
Conner clearly participated in the robbery of the two young victims. He was there in the planning stages of the trio’s agreed-upon robbing spree and aided Now-lin both during the crimes and after (when they attempted to conceal evidence that could point to their guilt). Because Conner participated in the underlying felony of robbery, the only remaining question is whether the murder was connected to the robbery.
Conner contends that the homicide was not causally related to the underlying robbery. His theory is that Nowlin killed the girl to prevent her from reporting that Nowlin had raped her. Since the underlying felony therefore must be rape, and there is no felony-rape statute in Iowa, the argument continues, Conner’s conviction under the Iowa felony-murder statute violates his right to due process under the United States Constitution.
This theory is unpersuasive because it is manifestly based on a misunderstanding of the Iowa felony-murder doctrine. The statute and the cases that have applied it do not refer to “causality.” Rather, as we discuss above, the law hinges on whether Conner participated in the initial felony, and whether it was connected with the murder. Clearly, the underlying plan to embark on a robbing spree with loaded shotguns (a plan that Conner participated in from inception to conclusion) was connected with the homicide for which Conner was convicted. The robbery set into mo*1388tion a continuous chain of events that soon culminated in the murder. Since we believe the conclusion that the homicide was committed within the res gestae of the underlying robbery is inescapable, we are unpersuaded by Conner’s interpretation of Iowa’s felony-murder statute.
C. The Jury Instructions
Conner also argues that the jury instructions used in the trial court failed to present his theory of defense and therefore deprived him of fourteenth amendment due process. However, as the district court points out, federal case law clearly establishes that improper jury instructions generally do not justify granting habeas relief.
The United States Supreme Court, in Henderson v. Kibbe, 431 U.S. 145, 154, 97 S.Ct. 1730, 1736, 52 L.Ed.2d 203 (1977), stated that “[t]he burden of demonstrating that an erroneous instruction was so prejudicial that it will support a collateral attack on the constitutional validity of a state court’s judgment is even greater than the showing required to establish plain error on direct appeal.” Id. at 154, 97 S.Ct. at 1736-37. See also Cupp v. Naughten, 414 U.S. 141, 146-47, 94 S.Ct. 396, 400, 38 L.Ed.2d 368 (1973) (in a habeas attack, it is not enough to establish that an instruction is undesirable, erroneous, or even universally condemned; rather, it must demonstrably have violated a right guaranteed by the fourteenth amendment).
In Williams v. Lockhart, 736 F.2d 1264 (8th Cir.1984), this court explained that ha-beas relief is available for faulty jury instructions only when “petitioner establishes that improper instructions resulted in a fundamental defect which inherently results in a complete miscarriage of justice or an omission inconsistent with rudimentary demands of fair procedures.” (Emphasis added). Id. at 1267. Williams emphasizes that petitioners must carry a “heavy burden” to establish that allegedly erroneous jury instructions rise “to the level of constitutional significance.” Id. See also Brouillette v. Wood, 636 F.2d 215 (8th Cir.1980), cert. denied, 450 U.S. 1044, 101 S.Ct. 1766, 68 L.Ed.2d 243 (1981) (allegation by petitioner of improper jury instructions without indication that a complete miscarriage of justice resulted do not generally form a basis for habeas corpus relief); DeBerry v. Wolff, 513 F.2d 1336 (8th Cir.1975) (claimed errors in instructions to the jury are generally not of such constitutional magnitude as to provide basis for habeas corpus relief); Dietz v. Solem, 640 F.2d 126 (8th Cir.1981) (same).
In light of this overwhelming weight of authority, the remaining question is whether the jury instructions in Conner’s trial were sufficiently improper to infect the entire proceeding and deprive him of constitutionally guaranteed due process. Our answer is that they were not. In fact, the instructions appear to us to have clearly and properly encapsulated the substance of Iowa’s felony-murder statute, which we discuss above.
D. The Brady Argument
Conner also argues that the state improperly suppressed exculpatory evidence during his trial. Apparently, Craig Christopher Sudduth, George Nowlin’s cellmate in jail, indicated in a written statement that Nowlin boasted in jail that he “was the one that killed the boy and girl and that Atwell Conner was just along at the time * * *. Nowlin told me that after he killed one of the two victims he pointed the gun at Conner and said he was going to kill him too.” Petitioner’s exhibit 28.
Conner did not learn of this statement until the discovery period in his post-conviction action even though the state possessed it during the trial.
The district court applied the test in United States v. Agurs, a key case in the progeny of Brady v. Maryland, 373 U.S. 83, 83 S.Ct. 1194, 10 L.Ed.2d 215 (1963), regarding state suppression of exculpatory information, to this issue and correctly concluded that no constitutional error was committed in withholding Sudduth’s statement.
In short, Agurs holds that constitutional error should be found only “if omitted evidence creates reasonable doubt or guilt *1389that did not otherwise exist.” United States v. Agurs, 427 U.S. 97, 112-13, 96 S.Ct. 2392, 2401-02, 49 L.Ed.2d 342. Under Agurs, the character of the evidence suppressed is to be examined, not that of the prosecutor. Id. at 110, 96 S.Ct. at 2400. The most significant factor to consider is the probative significance of the suppressed information. If it raises a reasonable doubt of defendant’s guilt, the suppression is constitutional error; if not, it is harmless error. Id. at 112, 96 S.Ct. at 2401.
In this case, the suppression is harmless. Sudduth’s statement is consistent with the state’s case in that it reiterates that Nowlin alone did the killing. The state agrees. It convicted Conner because he participated in the underlying felony of robbery which began a sequence of events that culminated in murder. Establishing that Nowlin was the murderer does nothing to refute the state’s position that the murder was committed within the res gestae of the underlying felony in which Conner participated. Therefore, the suppression of Sudduth’s statement did not deprive Conner of exculpatory evidence in violation of his constitutional rights.
III. CONCLUSION
Since the jury instructions were adequate, the Iowa felony-murder statute justifies Conner’s conviction, and the Brady claim does not raise any reasonable doubt of Conner’s guilt, the appeal is denied.
. The Honorable David R. Hansen, United States District Judge for the Northern District of Iowa.
. The factual setting of this case is set out in greater detail in State v. Conner, 241 N.W.2d 447, 450-51 (Iowa 1976).
. Former Iowa Code § 690.2. Chapter 690 of the Iowa Code was repealed, effective January 1, 1978, in the course of a reorganization of the criminal statutes of Iowa.
. Conner had requested instructions indicating that the State was required to prove a causal relationship between the underlying felony of robbery and the murder that ultimately occurred.
. See also Mitchell v. Wyrick, 727 F.2d 773, 774 (8th Cir.), cert. denied, 469 U.S. 823, 105 S.Ct. 100, 83 L.Ed.2d 45 (1984) (§ 2254 does not authorize review of state postconviction relief proceedings).