On April 28, 1969, four men robbed the Manufacturers Hanover Trust Company branch located at 748 Columbus Avenue, in Manhattan. Appellant Vaughan was eventually convicted after a non-jury trial before Judge John M. Cannella, in the United States District Court for the Southern District of New York, for violating, by his role in the robbery, 18 U.S.C. §§ 371, 2113(a) and 2113(d). Vaughan has never disputed his participation in the robbery, but rather has contended he was not sane at the time, because he had taken STP (a drug related to LSD) the evening before.
The principal issue in this appeal is whether Vaughan’s right to a fair trial and his waiver of a jury were impaired by the trial court’s continuous reference *93to, and apparent reliance upon, a letter written to the court by Vaughan immediately prior to trial, not introduced as evidence in the case, and which at the beginning of the trial the court said it would disregard.
Vaughan’s letter to the district court in part contained an offer to plead guilty to the conspiracy count on the understanding the trial judge would place Vaughan in a rehabilitation center operated by a court-appointed psychiatrist who had examined Vaughan.1 The district court explained to Vaughan on the day of the trial that it could not accept a plea because of the question of Vaughan’s responsibility for his actions. That is, under the circumstances, the Court did not wish to take the responsibility of accepting a guilty plea, but thought it more appropriate to consider all the testimony regarding Vaughan’s sanity at the time of the robbery. In assessing the voluntariness of Vaughan’s jury trial waiver, the court indicated that the letter would have no role in his ultimate decision:
“ * * * I will make every effort that I can to forget everything that is in this letter because when I try the case I can only decide the case on what appears in the evidence in the case, the exhibits and anything that happens during the trial, and I will not be able to render any decision based on any of the facts that you recite in your letter * * *”
Vaughan thereupon affirmed his election to waive a jury.
During the trial, two psychiatrists were called by the defense to testify. Both were court-appointed, one selected by the defense, and one by the prosecution. Each testified at length concerning his diagnosis of Vaughan's mental condition at the time of the robbery (based on statements made by Vaughan, including the claim he had taken STP before the crime was committed), as well as his condition at the time of trial. In a pre-trial report, Dr. Ralph S. Ban-ay, one of the psychiatrists, described Vaughan’s conduct at the time of the crime to have been the product of an “abnormal and unrealistic state of mind.” The other psychiatrist, Dr. Hugh F. Butts, found Vaughan to be “psychotic,” at the time of the crime. He also found that “[a]s a result of mental disease the defendant lacked substantial capacity either to appreciate the wrongfulness of his conduct or to conform his conduct to the requirements of law.”
In the course of the testimony of Dr. Butts, the court frequently asked him questions to clarify and expand various points. These questions reflected the trial judge’s deep concern and his careful weighing of the evidence. However, he asked Dr. Butts about the contents of the pre-trial letter written by Vaughan to him, using the letter, though it was not admitted into evidence, as an indication of Vaughan’s mental condition. For example, the judge said to Dr. Butts:
“What I want to ask you about is generally what that letter indicates. You see there is a bargaining feature to this letter. In other words, do these fellows that have these symptoms also become bargainers, they want to know if they do certain things, whether they can get something in return? In other words, he is offering to plead there to a conspiracy provided his conditions are met. This kind of bargaining, do they have this kind of a tendency, do they bargain with people in situations like this ?”
*94The court later handed Dr. Butts the letter from Vaughan, and the following colloquy then took place:
“The Court: It is the same kind of thing you elicited from him. The only reason I put it in is that it seems to me it covers a lot of the same ground that you covered and I got exactly the opposite view from it as you did.
The Witness: That strikes you as a very lucid letter.
The Court: Of course. Not only lucid, but the kind of a letter, with the education he has had, I don’t know how he did this. That is why I asked him whether he actually wrote this letter himself or whether somebody wrote it for him and phrased it for him. I can’t see how a fellow, if he has this psychotic condition that you are talking about, can put those thoughts together * *
After further testimony by Dr. Butts, the judge commented:
“I don’t see that it serves any purpose in going over this letter with you. I have read the letter a number of times. As I indicated to you before, I felt that this letter speaks loudly as far as he is concerned, more so than almost anybody could speak for him or tell about him * * * There are many impressions I gain from this letter.”
During Dr. Banay’s testimony, the court again referred to Vaughan’s letter as indicating the defendant’s great confidence in Dr. Banay:
“ * * * He wrote me a letter, for example, indicating that he was willing to plead to certain aspects of this case provided I place him in care of you and your rehabilitation center, where he would try to reorganize his life to cope with existing realities, and he wanted an opportunity to do that.”
At the close of the trial, the court rejected the opinions of the expert witnesses that Vaughan’s conduct at the time of the robbery was the product of a mental disease or defect, and found Vaughan guilty.
The Government contends that absent a showing of substantial prejudice, a court sitting without a jury is presumed to base its verdict only on proper evidence.2 Assuming the applicability of such a presumption, it is rebutted in this case by the trial judge’s repeated references to, and apparent reliance upon, a matter not in evidence. While we have no doubt that the court referred to the letter solely because of his conscientious desire to judge correctly this defendant, reliance on such extra-evidentiary matters is reversible error. Cf. United States v. Hamrick, 293 F.2d 468 (4th Cir. 1961). For a factual situation similar to the present case, see People v. Green, 21 Mich.App. 575, 175 N.W.2d 521 (1970).
The use of this letter as relating to defendant’s mental condition may not be considered harmless when such condition was the central issue in the case. The fact that the judge made no reference to the letter in his findings of fact cannot be determinative when there are continual references by the court to the letter in the record itself.
The Government contends that the absence of any objection by Vaughan’s counsel during the trial to the court’s references to the letter bars the defendant from raising the point on appeal. However, it would have been difficult at any particular time during the trial for defense counsel to know he should object once the trial judge had said he would not consider the letter. Preferably, counsel should have objected once the court’s reliance on the letter became apparent. Nonetheless, this error of the court may well have affected the integrity of the fact-finding process, and is therefore appropriate for appellate notice under Fed.R.Crim.P. *9552(b).3 As Professor Wright has stated, “It is not a miscarriage of justice to convict a guilty man, but if he is convicted in a way inconsistent with the fairness and integrity of judicial proceedings, then the courts should invoke the plain error rule in order to protect their own public reputation.” 4
Vaughan also urges that his conviction should be reversed because the government did not prove him to be sane at the time of the robbery. Vaughan’s theory is that a trier of fact may not disregard psychiatric testimony when no contrary expert testimony has been presented. See Beltran v. United States, 302 F.2d 48 (1st Cir. 1962). The psychiatrists’ testimony, however, was based on Vaughan’s statements to them that he had taken STP at the time of the robbery. Vaughan did not testify at trial. We have closely examined the record in this case, including the trial court’s findings of fact, and conclude that the court may not have accepted the factual predicate of the psychiatrists’ opinions that Vaughan had taken the drug at the time of the robbery. While the record is not wholly clear on this point, there is sufficient ambiguity to prevent us from holding the court erred in finding that the government had met its burden. Cf. United States v. Carter, 436 F.2d 200 (D.C. Cir. 1970).
The judgment of the District Court will be reversed, and the case remanded for a new trial.
. Vaughan’s letter, stated:
“I ask ‘you’ now for this plea of conspiracy with the understanding that I will not be returned to the animal environment of prison but placed in the care of Dr. Benet’s [sic] Rehabilitation Center, where I can try to reorganize my life to cope with the existing realities. I
ask you this in ernest [sic] and I hope you will be responsive to this plea.”
“Dr. Benet asked me, ‘What will you say to the judge’? Well your honor I have said it. I don’t want to beat a case. I want help, if justice or its purveyor can be responsive to another person. Please Help Me.”
. See United States v. Reeves, 348 F.2d 469 (2d Cir. 1965); cf. United States v. Bowles, 428 F.2d 592 (2d Cir. 1970).
. “Plain Error. Plain errors or defects affecting substantial rights may be noticed although they were not brought to the attention of the court.”
. 3 Wright, Federal Practice and Procedure, Criminal § 856 (1969); United States v. Atkinson, 297 U.S. 157, 160, 56 S.Ct. 391, 80 L.Ed. 555 (1936).