Congressional investigators are trying to determine why five top secret National Security Council documents were released from a special protective file last Nov. 21, routed to then-White House aide Oliver L. North, substantially rewritten at his direction and returned four days later without questions being raised about the alterations.

When the documents were returned, one had been reduced from four pages to one and all had been retyped on a new letterhead, according to information released last week by the House and Senate select committees investigating the Iran-contra affair.

However, the changes apparently were overlooked when the papers were checked back to the "System 4" master file of originals on Nov. 25, 1986, immediately after Marine Lt. Col. North's firing from the National Security Council staff was announced by President Reagan.

Congressional investigators are seeking to determine who ordered the release of the documents and why, sources say.

North's office files in Suite 302 of the Old Executive Office Building already had two copies of the documents. Two officials familiar with the handling of classified papers at the NSC agreed that it was highly unusual in such a case for originals to be sent without North offering an explanation of why he needed them.

"Normally, you wouldn't let go of the master copies," one former NSC official said. "Such a request would have been unusual, and they were not normally checked out of sight" of the custodian.

All five of the original documents were 1985 memos from North to former national security adviser Robert C. McFarlane dealing with U.S. support for the Nicaraguan contras while such aid was banned by Congress. The memos made clear that North was deeply involved in the war. They portrayed him as making proposals for sinking a ship carrying arms to Nicaragua, opening a "southern front" against the Sandinista government, and outlining a "fallback" plan for helping the rebels if Congress did not restore aid.

The altered versions deleted references to North's active role and made it appear that he was merely reporting on the situation in Central America.

According to congressional testimony last week by Fawn Hall, North's former secretary, North handed her the originals on the afternoon of Nov. 21 and told her to make the changes indicated in his handwriting. She testified that she made the alterations, destroyed the originals and made copies of the new documents on a letterhead that had not been in use in 1985; Hall said she intended to substitute them for the copies of the old originals in North's office files.

However, she testified, she was diverted when she went to help North shred a 1 1/2-foot stack of documents from his office safe, and did not remember the altered documents until Nov. 25. By that time, North had been fired and his office had been sealed at the direction of the NSC's Brenda S. Reger.

The altered originals were returned Nov. 25, according to a date stamp on a routing slip released by the congressional investigating committees. Hall, who testified that she was "emotionally distraught" at the time, proceeded to smuggle the copies of the altered papers out of the office while accompanied by North and Washington attorney Thomas C. Green.

A former NSC official knowledgeable about the records system described the sequence of events as "a little strange."

On Nov. 21, according to a paper released by the committees, six numbered documents from the master file of originals kept in the Intelligence Directorate near North's office were requested. All were in the System 4 series, which Hall said was reserved for the most sensitive papers handled at the NSC.

The newly appointed custodian of System 4 records at the time was Brian T. Merchant. The slip of paper indicated that the request for the documents may not have come directly from North, but went through the then-director of intelligence programs, Kenneth E. deGraffenreid. A note on the paper says, "Signed out to Ollie North, J.B." Hall identified J.B. as June Bartlett, deGraffenreid's secretary.

DeGraffenreid's superior was Vice Adm. John M. Poindexter, the national security adviser at the time, who resigned Nov. 25 shortly before Attorney General Edwin Meese III announced that North and Poindexter had known about a secret diversion of U.S.-Iran arms sales proceeds to aid the contras. According to North's calendar, he met with Poindexter at 1:30 p.m. on Nov. 21, shortly before the shredding began.

In a handwritten note on the list of numbered documents, Merchant wrote, "Please return these originals to me when you have finished." But it was four days before the altered documents were checked back; several officials said the relaxed signout procedure was a departure from the usual practice of rigorous supervision of highly sensitive NSC documents.

Merchant's handwritten notation said only five of the six requested documents had been found and that all copies of the sixth had been "destroyed." However, that document was found later in a file of 1984 papers, committee investigators said.

Sources said that at the time there was no official log of records signed out of the System 4 file, although one exists now.

Merchant declined through an NSC spokesman to be interviewed, and deGraffenreid, who no longer works at the NSC, would not comment.

The alteration of the documents did not become known until much later, when investigators noticed discrepancies between the copies of the same-numbered documents in North's files and those found in the System 4 master file of originals.