Efforts to stifle a federal investigation into then-White House aide Oliver L. North Jr.'s secret network aiding the contras began after the CIA station chief in Costa Rica tipped North off to the inquiries, according to newly released testimony and documents from the congressional Iran-contra committees.

Jeffrey Feldman, assistant U.S. attorney in Miami, described in testimony how he shocked then-U.S. Ambassador to Costa Rica Lewis Tambs on March 31, 1986, by showing him a chart he had drawn that put Marine Lt. Col. North at the top of a group that was supplying arms to Nicaraguan rebels at a time when Congress had banned U.S. military aid.

"The ambassador turned white," said Feldman, who added that Tambs immediately called in

the CIA station chief, who went

under the pseudonym Tomas Castillo.

Tambs and Castillo both testified last summer that at the time Feldman was meeting with them, they were involved in North's secret operation.

However, during the March 1986 meeting, which took place at the U.S. Embassy in San Jose, Castillo tried "to rip the credibility of the various people who were making the allegations," Feldman said in his deposition.

Immediately after Feldman left the meeting, Castillo telephoned North in Washington and told him of the visit and the investigation, according to a notation in North's notebooks dated March 31.

On April 4, Feldman returned to Miami and met with his boss, U.S. Attorney Leon Kellner, who asked that Feldman draft a memo about the case based on what had taken place in Costa Rica. One of Kellner's aides has told the committees and others that there was a call from a high-level Justice Department official during that meeting in which Kellner was told to "go slow," but others present have no recollection of such a phone call, and Justice Department officials deny it took place.

On April 12, Attorney General Edwin Meese III, on a visit to Miami, took a couple of minutes to ask Kellner about the investigation. Meese later told reporters he was "mainly curious" about what he had read in newspapers and was not concerned that the probe might hurt efforts to secure congressional funding of military aid to the contras.

A day or two later, Kellner said, Feldman returned the requested memo along with a recommendation that the case go to the

grand jury. Kellner said he sent the memo back for changes and that the process continued for several months.

Testimony and documents obtained last summer by the congressional Iran-contra committees portrayed members of the North network as taking other actions in this period to cover up evidence of the North-directed operation.

In April, North's associate in the contra operation, retired major general Richard V. Secord, hired a former CIA security specialist to investigate individuals in Costa Rica who were making allegations about North and his network, according to testimony to the Iran-contra committees last summer.

On April 7, North's courier to the contras, Robert W. Owen, reported to North on a trip to Costa Rica and, after repeating the story about the chart that included North's name, said Feldman was "not only looking at a possible violation of the Neutrality Act, but of possible unauthorized use of government funds."

"If and when I am contacted by the FBI I will not answer any questions without an attorney present. Even then, I will not answer any questions. It is the only way I can see to stem the tide," Owen wrote North, whose code name was "William."

Owen identified the investigation team as being headed by Feldman and including two FBI agents from Miami, Kevin Currier and George Kiszynski. The team went to Costa Rica to investigate an assassination threat against Tambs and reports of illegal gun-running to the contras from the United States.

According to Feldman's testimony, a U.S. Embassy employee told him that the National Security Council had contacted John Hull,

an American who had had ties to the CIA and was assisting the

contras, concerning the investigation.

Hull, who initially agreed to

be interviewed, subsequently changed his mind, Feldman said.

Feldman had drawn up a chart on the basis of what sources had told him concerning gun-running to the contras. The chart included North, Owen and Hull.

Feldman said that when he first called on Tambs to ask what he might know about a plot on his life, the former history professor laughed. "He thought that was a joke," Feldman said.

But when Feldman went on to show him the chart, "he obviously became distressed," and called in the top CIA hand. Feldman said he started taking notes and was told, "Don't take notes, don't take notes." Castillo, who testified in closed session before the Iran-contra hearings last summer, has been discharged by CIA Director William H. Webster. He is appealing the action.

It has not been established who in Costa Rica told Owen that he was one of those listed on Feldman's chart. None of those questioned admitted telling Owen that.

FBI special agent Currier said in his deposition, "We all thought we were on to something. We thought that the matter, the whole investigation should be pursued. We were all optimistic about returning to Miami and initiating . . . pursuing this matter aggressively."

Currier testified that after arriving back in the United States, the conversations with Tambs were reported to FBI headquarters in Washington. An earlier report in connection with another investigation had mentioned the names of Owen and Hull, he said.

There was, however, no grand jury action until six months later, after the Oct. 5, 1986, shooting down by Nicaragua of a contra-resupply plane operating under the supervision of Secord, with North's knowledge.