Sensitive FBI documents and the makings of a book about allegedly illegal FBI activities have been found among the ruins of the Peoples Temple cult in Guyana.

The discovery has triggered an investigation of the book's author, a former FBI agent who has been cooperating with the Church of Scientology in exposing purported FBI wrongdoing. It also has led to an exchange of accusations between Charles Garry and Mark Lane, attorneys for the Peoples Temple cultled by the Rey. Jim Jones, who died Nov. 18 with more than 900 cult members in a mass suicide-murder.

Justice Department and FBI spokesmen declined to comment on the matter yesterday. But in telephone interviews, the former agent and Garry, his attorney, said that FBI agents searched their offices a few weeks ago in an effort to recover other copies of the FBI materials found in Jonestown, Guyana.

The documents were said to be unrelated to the Peoples Temple.

Sources familiar with the investigation said the matter is considered serious because informants can be identified from some of the documents. One informant involved has received a death threat as a result, they said.

Possible charges against the former agent for stealing government documents may be barred. That is because of an understanding given by Justice Department investigators that he would be immune from prosecution for the alleged misdeeds he has described, sources said.

The department's Office of Professional Responsibility has interviewed the former agent about his accusations that the FBI routinely created phony informants -- and used illegal surveillance techniques, such as break-ins and wiretaps.

The former agent has given press interviews arranged by the Scientologists over the last several weeks. Curch officials have been under investigation for spying on federal authorities and have waged a public counteroffensive to discredit those investigators.

The former agent, who asked not to be identified, said in a phone interview yesterday that he first learned materials for his book were found in Jonestown about three weeks ago when six FBI agents from Los Angeles, armed with a search warrant, arrived at the boat where he lives.

They found no other documents and took only his typewriter, but told him he was under investigation for stealing government documents, he said.

He said he had sent the material to Garry's office more than a years ago to safeguard it until he could turn it over to Justice Department investigators.

He said he didn't know how the material got to Jonestown in South America.

Garry and Lane have wildly divergent versions of how the former agent's book material made its way from Garry's office in San Francisco to Jonestown.

Garry said he recently learned that Terri Buford, a key member of the Peoples Temple, had gone through his files about a year ago and copied the documents with the aid of another Temple member.

Garry was the Peoples Temple attorney at that time, he said, and had given Buford a key to his office. He said he gave the FBI the name of the Person who would corroborate the story.

Lane, Buford's attorney, said yesterday that a Garry associate gave his client the documents, with Garry's approval, to copy and give to both Jones and Dennis Banks, a leader in the American Indian Movement.

The two attorneys were present as Peoples Temple lawyers when Rep. Leo J. Ryan (D-Calif.) made the fateful trip to Jonestown in November. Ryan and four members of his party were murdered and then Jones led a mass murder-suicide in which he and more than 900 of his followers died.

Lane and Garry escped the massacre and have since become bitter enemies.

Both Garry and Patricia Richartz, Garry's associate, denied Lane's accusation. "I had no idea the douments were in Jonestown until the FBI came to search my office," Garry said.

The FBI agents took his copies of the documents found in Guyana, Garry said. He said he didn't know whether they included raw files.

Richartz said Buford knew about the former agent's proposed book because she had been introduced to him at Garry's office about a year ago.

The former agent said he retired in 1977 after 25 years in the FBI and decided to write a book because he wanted "to get a change in the system."

He said the documents included 212 pages of "working information" for a book about his knowledge of illegal FBI activities, as well as a few dozen copies of internal FBI documents he planned to turn over to investigators.

He said he didn't believe informants' names could be learned from the material, but noted that some of documents named FBI agents who took part in break-ins.

The former agent said he talked with Michael E. Shaheen Jr., head of the office of Professional Responsiblity, and his assistant, Richard M. Rogers, last spring and last October.

Both declined comment yesterday.

The former agent and Garry both said he had been told by Shaheen's office that he would not be prosecuted as a result of his testimony. Other sources agreed.