Former national security adviser John M. Poindexter's testimony, including his admission that he destroyed a signed presidential "finding" authorizing an arms-for-hostages deal, raises questions about whether he obstructed justice or participated in a criminal conspiracy, legal experts said yesterday. But they said the crucial question is whether, at the time Poindexter destroyed the document, he knew there was a criminal investigation under way and acted with intent to impede it.

They also said that Poindexter's destruction of the finding appears to violate a 1978 records preservation law that carries no criminal penalties but that could be used in building a case that Poindexter conspired to defraud the United States.

Poindexter's lawyer, Richard W. Beckler, said Wednesday that his client had been named a "primary target" of the federal grand jury investigating the Iran-contra affair and that the rear admiral faced the "imminent threat of prosecution." Congress voted to give Poindexter "use immunity" for his testimony, but independent counsel Lawrence E. Walsh may build a separate case against him.

Poindexter testified Wednesday that as the Iran-contra affair was unraveling last November, he destroyed a 1985 presidential finding authorizing the arms-for-hostages deal in order to "protect" the president from "a significant political embarrassment." He also testified that he believed that Attorney General Edwin Meese III, who questioned him about the Iran-contra affair, was acting "in his role as really a special adviser to the president."

He testified yesterday that last October, after Eugene Hasenfus' airplane was shot down in Nicaragua and the administration's role in the contra resupply network began to emerge, he told NSC aide Kenneth DeGraffenreid that North's computer memos about the contras "probably did reveal too much" and "that we probably would be better off without those memos."

The FBI began an investigation of the Hasenfus incident three days after the plane was shot down. It was not clear from Poindexter's testimony yesterday when in October the conversation with DeGraffenreid occurred.

"Obstruction of justice requires that there be some kind of proceeding already commenced," said Columbia Law School Prof. John C. Coffee Jr. "It is not illegal to destroy an incriminating document. It is only obstruction of justice as of the time when the proceeding has begun."

Thomas P. Puccio, the federal government's principal prosecutor in the Abscam influence-pedding affair, said the central question is Poindexter's mental state.

"You have a situation where, fearing that investigators who are conducting an authorized investigation are going to find something that you don't want someone to find, you order something destroyed. That's something which can be viewed as obstruction of justice," said Puccio, now in private practice in New York.

However, he said, "If you do something without the intent to defeat an investigation but because it would be embarrassing to someone or because you're well within your authority to destroy documents, that's a horse of a different color."

Lawyers familiar with the Presidential Records Act, a 1978 law governing preservation of White House documents, said Poindexter's destruction of the 1985 finding apparently violated the statute.

Like the Boland Amendment cutting off funds to the contras, the Presidential Records Act does not carry criminal penalties. However, lawyers said, Walsh could argue that Poindexter's violation of the law was part of a conspiracy to defraud the United States of a faithful execution of the law.

Poindexter testified that he was "aware" of the law but "did not think about that at the time" he tore up the finding and put it in a "burn bag."

The law, passed after a struggle for control of former president Richard M. Nixon's papers and tapes, provides that presidential records -- those "created or received by the president" or his staff and which involve the execution of the president's duties -- may be disposed of only with the approval of the U.S. archivist and if they no longer have "administrative, historical or evidentiary value."

"The concern was that documentary history would be destroyed," said a congressional staffer who worked on passage of the law. "This is a classic example of why the law was passed."

The staffer said the law contains no penalties because "it was tough enough getting it on paper and in law that the records were the property of the citizens of this country."

Coffee said the general conspiracy to defraud the U.S. statute "permits the government to charge you with conspiring to do something against the United States that isn't criminal" in itself. The law, Coffee said, has been used to cover "almost anything that interferes with the administration of a government agency."

Some lawyers questioned whether it would be wise for Walsh to use obstruction of justice or conspiracy laws to punish conduct that is not itself criminal.

"Any prosecutor can take the general conspiracy laws or general obstruction laws and make them apply to this case," said Abbe Lowell, who has represented a number of public figures under federal investigation.

But, he said, "People obstruct justice usually to cover their tracks in a criminal scheme. What still has not been revealed here is: Is the underlying act criminal? Is sending arms to Iran criminal? Probably not. Was sending funds to the contras criminal? At worst it violated the Boland Amendment . . . If there was no criminal scheme with which to begin, the obstruction shouldn't survive in a charge unto itself."