The Justice Department yesterday asked the FBI to start an investigation of possible jury tampering because of reports that the lone holdout in the mistrial of Rep. Daiel J. Flood (D-Pa.) tried to influence other jurors with information from a "secret" outside source.

Philip B. Heymann, head of the department's Criminal Division, said the investigation was being ordered "in the best interest of the administration of justice."

The 11-count bribery and perjury trial against the ailing 75-year-old congressman ended in a hung jury last Saturday when one juror, William Cash, refused to agree with his colleagues that Flood was guilty on any of the counts.

Jurors said later, in reports first published by The New York Times, that Cash had told the other jurors that prosecution witnesses had stolen $176,000 from Flood.

No such evidence was introduced at the trial.

But sources said later that Cash's claims were similar to accounts that a key witness, Stephen B. Elko, had made to FBI agents a year ago during the Flood investigation.

Cash later told U.S. District Court Judge Oliver Gasch that he made up the story as a "joke." But the judge told The Washington Star yesterday that the incident is "past the point where it's a joke." The investigation of possible jury-tampering would be desirable, the judge was quoted as saying.

Axel Kleiboemer, Flood's defense attorney, said last night of the announced FBI inquiry: "It's just terrible that something like this comes up when the possibility of a second trial is so real.

"The full force of publicity on this can have a bad influence on the decision-makers at the Justice Department [on whether to seek a new trial]. And it could taint the pool of jurors for a likely second trial."

Heymann's statement yesterday said a decision on seeking a retrial would be delayed, but Justice sources have said such a request is almost certain because of reports by other jurors that Cash alone prevented guilty verdicts on some of the counts against Flood.

The congressman was charged with trading his influence as chairman of a powerful House appropriations subcommittee for cash and bank stock. The money allegedly was usually funneled through Elko, the chief prosecution witness, but several other witnesses testified that they made direct personal payments to Flood.

Kleiboemer argued that Elko had taken advantage of his elderly employer and kept the money for himself.

According to several jurors, Cash, a career Navy man in his 60s, said he would never vote to "send that old man to jail."

Juror Elizabeth Vegos said, "I as well as others were pressing him to explain to us why he thought Mr. Flood should be acquitted. Finally, he just burst" and told the others he had "information we didn't have from a secret source."

The information was that Elko, along with others, had somehow stolen $176,000 from Flood in California. Cash also made reference to a $47,000 check being involved in the allegation.

Cash told the Star yesterday that "I don't know where I came up with the $176,000 figure."

Kleiboemer said last night that he sill has no idea what the $176,000 refers to because he has never been given any such FBI interviews with Elko by the prosecution.

The jury was kept in a hotel throughout the trial, without access to newspaper accounts of the trial or television news.

The telephones in the rooms were made inoperative, and jurors were summoned to a suite of U.S. marshals for any emergency calls. Those calls, according to jurors, were monitored by marshals to guarantee that no information related to the trial was discussed.

Outsiders were kept from the jurors in the hotel, where the panel members lived in a wing sealed off by marshals, and in the private dining rooms at the hotel and the courthouse.

Gasch said that had the jury not been sequestered, he would have ordered an immediate jury tampering investigation when he discovered Cash's activities.

But he said he was assured throughout the trial that there were no breaches of security.