The question presented is whether the giving of a second modified Allen charge,1 in response to a jury’s report of deadlock, has such coercive implications that it should, as a sound rule of practice, be prohibited. We conclude that it has.
Seawell was convicted for armed bank robbery and the use of a firearm hi its commission. He was found to be one of three masked robbers and, specifically, the one who guarded the front door of the bank during the robbery. His trial lasted four and one-half days, from Monday to Friday, in the course of which the jury heard the testimony of twenty-two witnesses. There followed closing arguments and the court’s instructions. The jury then deliberated for two hours on Friday afternoon before recessing for the weekend.
On Monday, two notes were sent to the judge after the jury had deliberated approximately one and one-half hours. The first note said, “[t]he jury is at a ten-to-two impasse. The two state that nothing we can say will convince them otherwise. What course of action should we now take?” The second note, received minutes later, asked that a certain witness’s testimony be read. Assuming that the second note had superseded the first, the judge had the testimony reread and then indicated to counsel his intention to give a modified Allen charge if the jury remained unable to agree.
Ten minutes after resuming deliberations, the jury sent another note indicating *1161a ten-to-two deadlock, and a modified Allen charge was given.2 Approximately three and one-half hours later, after some intervening discussion between judge and jury, *1162the judge received another note from the jury which read in part:
“No amount of argument has persuaded their convictions, these are the others who do not agree with the majority of the jurors. We therefore submit to you that we are at an impasse and are not likely to change our minds until fatigue becomes a deciding factor which we believe is neither fair to the defendant or the people.”
The court, after stating that the jury would not in any event have to deliberate later than 6:30 P.M. that night or beyond the point of fatigue, then reread the Allen charge. The jury retired at 4:10 P.M. and the guilty verdict was returned at 5:00 P.M. Timely objections were made by defense counsel.
Problems arising from the inherently coercive effect of the Allen charge have caused other courts of appeals3 and state courts4 to prohibit or to restrict severely its use. Nevertheless, the content,5 timing6 and circumstances surrounding7 the Allen charges given here have been upheld by this circuit and we do not now undertake to re-examine those decisions. We have, however, recognized that even in its most acceptable form, the Allen charge “approaches the ultimate permissible limits.” Sullivan v. United States, 414 F.2d 714, 716 (9th Cir. 1969). We conclude that permit*1163ting it to be given twice in a federal prosecution would be an unwarranted expansion of its use.
If the charge is to pass muster as instruction on the law there is little need to repeat it save at the jury’s request. (Here, for example, it was repeated three and one-half hours after it was first given. Nothing had intervened to cause the jury to overlook what last had been said to them.) Repetition of the charge, together with rejection of the jury’s second report of deadlock, is almost certain to convey the thought that by failing to come to an agreement — by once again reporting themselves at impasse — the jurors have acted contrary to the earlier instruction as that instruction was properly to be understood. (“Apparently you didn’t listen to what I said before, so I’ll repeat it.”) Given a second time, not at the request of the jury but at the instance of the judge, the charge no longer serves as an instruction; no matter how it may be softened it becomes a lecture sounding in reproof.
Ordinarily, the general test of whether a supplemental jury instruction is in error is to consider all the circumstances to determine if the instruction was coercive. Jenkins v. United States, 380 U.S. 445, 446, 85 S.Ct. 1059, 13 L.Ed.2d 957 (1965). Pragmatic considerations weigh against the application of this test when an Allen charge is given more than once. A case-by-case determination would provide little, if any, guidance for a trial judge. Defendants would also face insurmountable difficulties in attempting to show prejudice. A single Allen charge, without more, stands at the brink of impermissible coercion. We believe that the protection of a defendant’s right to an impartial jury compels a per se rule.8 Such a rule is not at odds with prior decisions of this court9 or other courts of appeals.10 We conclude that as a sound rule of practice it is reversible error to repeat an Allen charge in a federal prosecution in this circuit after a jury has reported itself deadlocked and has not itself requested a repetition of the instruction.11
*1164Since remand for new trial is necessary we deem it advisable to settle an additional question that may again present itself below. Appellant asserts that a delay in his indictment violated his due process guarantees. The robbery occurred on November 18, 1974, and Seawell was arrested December 4, 1974. Nevertheless, Seawell was not indicted until May 28, 1975, more than six months after the bank robbery and five and one-half months after his arrest.
In United States v. Marion, 404 U.S. 307, 324, 92 S.Ct. 455, 465, 30 L.Ed.2d 468 (1971), the Supreme Court stated that:
“ * * * the Due Process Clause of the Fifth Amendment would require dismissal of the indictment if it were shown at trial that the pre-indictment delay in this case caused substantial prejudice to appellees’ rights to a fair trial and that the delay was an intentional device to gain tactical advantage over the accused.”
We need not decide whether substantial prejudice and intentional delay are required in the disjunctive or conjunctive.12 Appellant has not even attempted to argue that delay on the part of the United States was an intentional device and we reject his claims of prejudice. His defense was alibi; he claimed he was working at a gasoline station at the time of the robbery. Two witnesses testified that he was there that day but could not place him there at a time that would have made his alibi air-tight. Appellant argues that but for the delay the memories of his witnesses would have been sharper, to his advantage, and that he himself might have remembered other witnesses.
The unsupported claim that memories of some witnesses and the defendant have dimmed does not constitute substantial prejudice in itself. Mere speculation cannot serve as the grounds for a finding of substantial prejudice. See United States v. Griffin, 464 F.2d 1352, 1354-55 (9th Cir. 1972), cert. denied, 409 U.S. 1009, 93 S.Ct. 447, 34 L.Ed.2d 302 (1973); cf. United States v. Finkelstein, 526 F.2d 517, 526 (2d Cir. 1975), cert. denied, Scardino v. United States, 425 U.S. 960, 96 S.Ct. 960, 48 L.Ed.2d 205 (1976). We conclude that appellant’s due process claim based on a delayed indictment is without merit.
Reversed and remanded for a new trial.