The burial heap of J. Edgar Hoover's misdeeds seems well nigh bottomless. Now ABC News has applied its redoubtable steam shovel to the excavation, unearthing a pile of muck in which several fragments of new and (to the still-shockable) shocking material may be espied.

From tonight's "ABC News Closeup" with Marshall Frady (Channel 7 at 10), we learn that Hoover knew (and failed to tell the Warren Commission) about various hints of Cuban, Soviet and/or Mafia involvement in the assassination of John F. Kennedy--most notably, about an informant's report that two months before the assassination, Lee Harvey Oswald went to the Cuban Embassy in Mexico City and offered to do the deed for Cuba. That tale is attributed to no less a source than Fidel Castro, who supposedly told it to an FBI informant in 1964.

What, if anything, should be made of this? Why was Hoover so reticent about relaying these tidbits to the Warren Commission? Frady suggests it was because the FBI ought to have been more worried about Oswald before the assassination and because, after the fact, Hoover wanted to cover up an "intelligence disaster." Yet there are other possible explanations. As the ABC program also notes, there was a history of CIA plots against Castro's life, which Hoover also knew of and kept from just about everybody, including his own agents. Even those who entertain doubts about the virtue of this particular CIA activity would probably agree that if it is going to be attempted, it had better be done on the sly.

There is also the possibility that Oswald's dealings with Cuban and Soviet operatives were a one-way street--that he offered his services but was never taken up on the offer. If so, and if Hoover and other highly placed Americans believed it was so, they might have had good "national security" reasons, from their perspective, for trying to keep Oswald's Cuban-Soviet connections under wraps. They might not have wanted "to inflame the American public and thus possibly cause World War III," as former FBI agent James Hosty (one of 17 Dallas agents secretly disciplined by Hoover for pre-assassination negligence) puts it.

The large and murky subject of the Kennedy assassination occupies about 10 minutes of tonight's hour-long program--a space of time that is at once hopelessly inadequate and (for those who have tended to lose interest over the last 19 years) more than enough.

The other major revelation dug up by Frady and off-camera investigator Patricia K. Lynch involves the guilt of Julius and Ethel Rosenberg. According to another ex-FBI man, Robert Lamphere, the Rosenbergs' atomic espionage activities were gleaned from secret KGB messages never introduced at the trial because U.S. intelligence officials didn't want the Soviets to know their code had been cracked.

This has a sensational ring, but like much of tonight's program, it raises quite as many questions as it answers. What is the basis for Lamphere's claim? Did the KGB messages squarely implicate the Rosenbergs or merely help form a mosaic of evidence against a larger group of conspirators? How, if at all, does this bear on whether the Rosenbergs were serious spies who helped the Russians get the bomb, or naive amateurs whose information was too vague to be of any value?

Rarely has an hour-long documentary tried to cover as much ground as this one--and now, perhaps, we know why others have been less ambitious. Frady and Co. have crammed so much into their time slot that, evidently, they couldn't find room to perform such basic journalistic duties as explaining who their various witnesses are and what axes, if any, they might have to grind. (It might, for example, be relevant to mention just why Hosty was labeled negligent, or that former National Security Council member Morton Halperin, also heard from in the program, was a victim of the Nixon-Kissinger wiretaps).

The general sketchiness of the documentation and the use of second- and third-hand testimony from highly interested parties leaves a large credibility burden on correspondent Frady. As an on-the-air personality, unfortunately, he comes across as something of a G-man himself, rat-a-tat-tatting his way through the Hoover history and piling evil upon evil to a fittingly melodramatic musical accompaniment.

So there is a certain preaching-to-the-converted flavor about of the program. But the aroma of the underlying subject matter comes through with enough pungency that the unconverted, too, will probably get a good whiff. And when so many noses have been so badly deceived for so long, this may be a valuable service.

The program is at its most powerful, strangely, side of Hoover's rule. When former New York City police commissioner Patrick V. Murphy tells how the FBI would routinely take credit for solving cases "on which the local police had done most of the work," and when the late William Sullivan, Hoover's longtime deputy, tells how Hoover once shared an elevator with a pimply faced, red-vested employe, and promptly had him fired on the principle that "we're not going to have anybody working for us who wears a red vest and has a pimply face," the full dimensions of Hoover's derangement are revealed, and the more appalling charges seem, if not provable, at least plausible.

And a question surfaces that may, in the end, be as important as any of the sexier stuff about assassinations and atomic spies. When a public official proves as cruel and quirky in his everyday dealings as, by all accounts, Hoover was, why should we expect him to be any better-balanced in his judgments about questions of national policy?