The Justice Department is investigating the possession of voluminous files of top-secret intelligence documents the FBI recovered last January from the home and automobile of former U.S. ambassador to South Vietnam Graham Martin.

The documents, according to one knowledgeable official, include highly classified CIA communications between the former U.S. embassy in Saigon and Washington.

They cover the entire span of major U.S. involvement in the Vietnam conflict from 1963 to 1975. As one official put it, "hardly anyone who played an important role in the Vietnam drama" was omitted from the classified reports in Martin's possession. "It includes the entire time of our viceroyship in Vietnam," he said.

The Justice Department has been considering whether to prosecute Martin under a seldom-used statue dealing with the "misuse" of classified documents. Officials at the State Department could cite no evidence that Martin had permission to keep the documents, which are presumed to have been taken from the files of the Saigon embassy, where he served from July 1973 until the American evacuation April 30, 1975.

Former CIA officer Frank Snepp, who wrote a highly critical book about his service in Vietnam, said in a phone interview last night that he had told the CIA and the Senate Intelligence Committee in 1977 that Martin took secret documents with him in the evacuation from Saigon.

"He told me he kept them so he could have the last word on (former Secretary of State Henry A.) Kissinger," Snepp said.

None of the authorities he informed of Martin's actions expressed concern or interest in doing anything about the matter, Snepp said.

Martin, interviewed by telephone at a Winston-Salem, N.C., hospital where he is recovering from lung surgery, said he had intended to turn over the papers to the Lyndon B. Johnson Library in Austin, Tex.

"I thought the historians might be able to do a better job at telling the story than has been done so far. They should have a shot at it," said the former ambassador.

But Martin did not call the LBJ Library to volunteer his papers until several weeks after North Carolina State Police found a batch of documents stamped "top secret" in the trunk of his car and called the FBI in on the case. The accidental discovery was made by police in the course of recovering Martin's car after he reported it stolen.

The discovery of the large cache of highly secret documents was made by the state police last January when they found several cartons of papers both inside the car and scattered around it.

State and Justice Department officials familiar with the case said they had no knowledge of Martin's motives in keeping the documents, which are assumed to have been in his possession since the evacuation of the American embassy nearly 3 1/2 years ago.

After the discovery, the documents were transferred to the Greensboro, N.C., office of the FBI. Martin says he also has retained some documents in his home which he intends to annotate and deliver to the LBJ Library. The former ambassador said he regards all the Papers as his personal possessions.

In addition, Martin has told officials of the LBJ library that his proposed gift includes a third category of documents that currently are being held in two security vaults at the State Department in Washington and that, according to Martin, are to be turned over to the library after joint screening by him and department officials.

Harry Middleton, director of the LBJ Library, said Martin contacted him last Feb. 13 and notified him of his decision to deposit papers in the archive. He said Martin told him of the circumstances of the car theft the preceding month and that the FBI had taken custody of portions of his proposed bequest.

Middleton said he was not aware that a criminal investigation was being conducted into Martin's possession of the documents. "We had been going on the assumption that the library would get the documents after the FBI, and I presumed the State Department, made the necessary determinations about what we were entitled to receive."

State Department officials involved in the case said yesterday they had never heard in its eight-month history that Martin intended to bequeath the impounded documents to the LBJ Library.

Middleton said he prepared a deed of gift which Martin signed on Feb. 22. The LBJ Library director then forwarded it to James B. Rhoads, archivist of the United States in Washington. Rhoads, in turn, said he routinely accepted the transfer to the library - a procedure required in all such transfers to presidential libraries.

"Normally we leave it up to the directors of the presidential library involved to satisfy themselves that the gift is in order," Rhoads told The Washington Post last night. He added that he was unaware of any criminal investigation into the circumstances of the classified material. "Normally I don't get into that sort of thing unless there is some problem that is called to my attention and that requires checking," he said.

Justice, State and FBI spokesmen all declined to comment on the Martin affair.

It was learned, however, that the content of the find included some of the most sensitive documents that can pass through any embassy - back-channel CIA cables which bypass the State Department's traffic network and personnel, sometimes including the ambassador.

Unlike the cases of Frank Snepp and Daniel Ellsberg, central figure in the Pentagon Papers case, as well as other dissident former officials, there is no evidence that Martin intended to publish the classified material.

In the telephone interview yesterday, Martin said. "I have written nothing about Vietnam. I have said nothing about Vietnam . . ."

In a March 1977 interview with Martin, the Chicago Daily News quoted him as saying he "might" write a book about his Vietnam experience. The Daily News said Martin spoke of having brought out of Saigon copies of exchanges of messages with Kissinger and adding: "I was looking at them the other day. They make mighty interesting reading."

The episode of the documents was the latest in a series of unhappy developments which have afflicted Martin since his photograph was flashed round the world during the final collapse of American involvement in Vietnam. He was unable to get reassignment within the State Department and later retired. His management of the final weeks of the evacuation has been criticized within the government as well as by such outside critics as former CIA intelligence officer Snepp.

From all the evidence yesterday it appeared that the Martin case was being handled at low level within the Justice Department's criminal division. Deputy Attorney General Benjamin R. Civiletti, who oversees criminal investigations and prosecutions, said yesterday in Salt Lake City, Utah, that he was unaware of the matter.

Martin began his State Department career in 1947. He held several major assignments, including the ambassadorships to Thailand and Italy, before going to Vietnam.