President Reagan was told in a secret memo written by Marine Lt. Col. Oliver L. North last year that Jack Terrell, a self-described mercenary who had become a vocal critic of the Nicaraguan contras, was a "terrorist threat" under investigation by the FBI, according to documents released by the congressional Iran-contra panels.

The documents have added fuel to allegations by administration critics that North, the National Security Council aide fired in the Iran-contra affair, used the Federal Bureau of Investigation to discredit opponents of Reagan's contra program. Sources said congressional Iran-contra investigators so far have not been able to substantiate such charges.

Documents show that at the time of the memo to Reagan, the FBI was investigating Terrell for allegedly threatening the president. The investigation ended and no charges were brought against Terrell, who denies that he threatened Reagan.

An FBI official said the bureau's probe of Terrell began independently of North and was prompted by what appeared to be a "very real, legitimate threat." The official said the inquiry had "absolutely nothing to do" with Terrell's anti-contra views.

According to documents released by the congressional panels, North knew of attempts by members of his private contra support network to uncover derogatory information on Terrell. Glenn A. Robinette, a former Central Intelligence Agency employe working with North's network, met with Terrell and described him as "extremely dangerous," according to a memo by North.

North outlined his concerns about Terrell in memos he prepared in July 1986 for then-national security adviser John M. Poindexter and for Reagan. The president read and initialed one of the memos. "It is important to note that Terrell has been a principal witness against supporters of the Nicaraguan resistance both in and outside of the U.S. government," North said in the memo sent to Reagan.

Testimony and documents from the Iran-contra investigations have outlined a concerted effort led by North to shield his secret network from scrutiny by Congress and the public. Among other things, North and others misled Congress about the extent and nature of North's contra activities. The recently released documents add a dimension to these disclosures, showing that North was also worried that Terrell might provide information to Congress and the press that would expose the network.

Robert W. Owen, a member of the private network, said in a January 1985 memo to North that Terrell "knows too much and it would do no one any good if he went to the press." He said Terrell "has got to be finessed out."

By July 1986, Terrell had become a source of critical information about the contras to congressional staffers looking into possible illegal White House support of the rebels. Terrell was also providing information to private groups opposing Reagan's Central America program, and to journalists. Less than a month before the FBI began investigating him on the alleged threat, Terrell had appeared on a CBS "West 57th Street" broadcast that in part examined North's secret involvement with the contras during a two-year prohibition on most U.S. military aid to the rebels.

According to North, after the CBS broadcast Robinette was asked to look into "how much Terrell actually knows" about the secret contra network. North did not name Robinette but referred to him in the memos as a security official for "Project Democracy," the name North sometimes gave to his secret network headed by retired Air Force major general Richard V. Secord.

Poindexter told congressional investigators in his private deposition July 2 that he gave Reagan the Terrell memo because of the alleged threat on Reagan's life.

However, a White House source said that it is highly unusual for Reagan to be informed about assassination threats. In addition, the memo Reagan received is largely devoted to North's account of Terrell's anti-contra activities and includes the comment that private contra supporters had been "particularly helpful" to the FBI in the Terrell investigation.

A knowledgeable congressional source said investigators remain puzzled by Poindexter's decision to inform Reagan about Terrell's activities, particularly in light of statements by Poindexter and other aides that they generally spared Reagan intricate details of his policy initiatives. "There certainly is a question about why they would bring this kind of thing to the president," the source said.

Terrell, who now works for the International Center for Development Policy, an anti-contra group headed by former U.S. ambassador Robert White, said in a recent interview that the FBI investigation was part of a "massive campaign" to discredit him and others.

"What leaps out from the {North} memo to me is that you've got 'Project Democracy' functioning as a domestic surveillance organization," Terrell said.

Secord has said that Robinette was hired to investigate the backgrounds of those behind a civil lawsuit filed in May 1986 against Secord, North and many others. Secord and North were concerned that the suit, filed by the Christic Institute, a liberal church-funded law group, might expose their secret contra support network. Secord said one person Robinette was asked to investigate was Terrell, whom Secord and North expected to be a "star" witness in the suit, brought months before the Iran-contra scandal erupted.

In a July 17, 1986, memo, North said that Oliver (Buck) Revell, FBI executive assistant director for investigations, had contacted him that day for information on Terrell in connection with the FBI's investigation into the alleged assassination threat and that North had arranged for Robinette to meet with FBI agents.

That same day, before he met with the FBI on Terrell, Robinette conferred with North and gave him copies of documents he had gathered and also later turned over to the FBI, according to an FBI report on an interview agents had with North about Terrell.

An FBI official said that Revell was seeking information about Terrell from several well-placed U.S. officials and that North was an obvious person to contact. The official said that presidential threats are treated as a "high-priority" investigation and Revell "reached out for whoever he could."

Before the bureau began investigating Terrell for the alleged threat, according to a North memo, Terrell beginning in March 1986 had cooperated with the FBI in a separate investigation into, among other things, possible illegal shipments of arms to the contras by supporters in the United States.