The Justice Department declared its intention yesterday to find out exactly how and why more than 900 Americans died in the jungle of Guyana and whether any Peoples Temple church survivors there or in the United States pose a danger to other Americans.

"We want to know who led it, who planned it and how did these events come about," said Assistant Attorney General Philip B. Heymann, referring to the forced mass suicide of 911 American members of the Peoples Temple in Jonestown, the murder of Rep. Leo J. Ryan (D-Calif.) and four others at a nearby airstrip, and the murder of four other temple members in the capital city of Georgetown.

The Justice Department's legal jurisdiction is limited specifically to trying to find and prosecute Ryan's murderers, because of a statute that makes conspiracy to murder a congressman a federal crime. But Heymann told reporters at a briefing yesterday that the investigation also will try to determine whether there is "any future danger for American citizens," from surviving cult members.

The FBI is checking reports that members of the cult once compiled a "hit list" of prominent politicians and journalists to be kidnaped or assassinated if something happened to the cult, Heymann said. But he cautioned that some of the allegations were "dated," and said at one point, "I don't want to suggest we have a valid list."

Justice Department briefings about ongoing investigations are rare, but officials said yesterday's unusual session was held because of the intense public interest and voluminous press inquiries about the federal investigation of the massacre in Guyana. At one point, Heymann said that Justice wanted to help answer the question of "how 900 people can be made to kill themselves."

The Justice Department also revised the total number of bodies of American dead airlifted from Guyana to the U.S. Air Force base at Dover, Del., to 911. There is relatively little hope that many more survivors will be found.

Guyanese police sources had said in Georgetown last week that a Venezuelan law enforcement official passed along to them a report from a pilot of a Venezuelan border patrol plane who said he saw perhaps 30 to 40 people in the rain forest in Guyana heading toward the Venezuelan border. However, officials here and in Guyana and Venezuela said yesterday they knew nothing about any such sighting and are not aware of any evidence of possible Jonestown survivors in Venezuela.

Among the other questions that remain unanswered is how many of cult leader Jim Jones' top lieutenants, many of whom were armed security guards in Jonestown, are among the survivors.

Guyanese authorities believe they have under arrest the only survivor among the Jonestown gunmen who ambushed Ryan and his party at the Port Kaituma airstrip. That man, Larry Layton, 32, from San Francisco, has been charged in court in Guyana with the five murders.

If Layton is put on trial there as expected, he could not be extradited he could not be extradited later to face more charges in the United States, because of the terms of the extradition treaty between the two countries, Heymann said. "We will do anything in our power," he added, "to assist Guyana in its investigation."

Earlier attempts to send a team of FBI investigators to Guyana were rejected by the Guyanese government. But the State Department now has arranged for four FBI evidence specialists to fly there, sources said. They were expected to arrive in Georgetown last night.

One Justice Department source said the FBI agents will photograph and examine the scene of the murders and suicides but will not interview survivors or suspects in the murders.

Heymann also said yesterday that the U.S. government has decided to perform autopsies on four random victims of the forced mass suicide to determine the cause of death. The Guyanese government earlier had asked for an autopsy of Jim Jones, and the Justice Department decided too, to examine the bodies of Jones' mistress, and -- after a request from his parents -- the doctor Lawrence Schacht, who mixed the batch of poisoned soft drink.

None of the other five or six gunmen identified by survivors of the airstrip massacre is known to be alive, according to the Guyanese authorities. But Michael Abbell, the Justice Department attorney coordinating the U.S. investigation, said Wednesday that thus far just two of the gunmen have been identified as being among the dead.

Heymann, who said repeatedly yesterday that he did not want to discuss specifics of the U.S. investigation because that might hinder future prosecutions or the parallel probe in Guyana, added "a number of [the airstrip shooting suspects] will probably have turned out not to have survived."

Peoples Temple survivors have said they also fear others among the survivors, especially those still under house arrest by the Guyanese authorities in the cult's Georgetown headquarters house. Jim Jones' 19-year-old son, Steve, and other members of the Jonestown basketball team who are inside the house have been accused by other survivors of being trained, sharpshooting members of the Jonestown security force.

One resident of the Georgetown house, Charles Beikman, 43, from Indianapolis, has been charged with the murder of fellow temple member Sharon Amos Harris and her three children, who were found inside the house with their throats slashed on the night of the forced mass suicide in Jonestown. Guyanese police are still investigating whether others in the house also were involved in the murders.

The deputy police commissioner in Guyana, Cecil A. Roberts, told reporters there yesterday that most of the survivors in the house and those staying under police guard in a Georgetown hotel have been cleared of involvement in any crimes. He indicated that they may be released to return to the United States as soon as this weekend.

He said all the survivors have been interviewed to determine whether any more should be detained as suspects or witnesses. Those interviewed, Roberts said, described at least three "dry run" mass suicide rehearsals in which Jonestown residents drank liquid they were told was poison.

Roberts said he has been unable to determine from the survivors or other evidence exactly how Jim Jones was shot to death. The cult leader's body was found on the platform of the commune's assembly hall in front of his green, high-backed wooden throne chair. A handgun, Roberts said, was found "nearby."

An autopsy of Jones' body could help answer the question of whether he or someone else fired the shot that killed him. It also could provide clues about physical illnesses Jones apparently was suffering from and whether he was drugged.

"We had been divided about doing autopsies," Assistant Attorney General Heymann said yesterday. "I myself wanted us to take any step -- even those that in another situation might have seemed excessive -- to nail down all the available evidence in this case."

In Guyana, Roberts said authorities there consider the deaths of all the children fed poison in Jonestown Nov. 18 to be murder. Adults forced against their will to take the poison also should be considered murder victims, Roberts added. Such distinctions could matter when relatives of the dead make insurance claims or file suit against what remains of the Peoples Temple.

Roberts also said police in Guyana found less than $1 million in U.S. and Guyanese currency in Jonestown and found no valuables in the heavy suitcase that three of the survivors said they were given by Jim Jones' mistress to take to the Soviet embassy. The three, who have been questioned closely by Guyanese police, said the suitcase contained gold and jewelry besides the cash found on them when they were picked up by police after abandoning the suitcase, which they found too heavy.

Guyanese police also found 32 firearms and several crossbows in Jonestown, according to Roberts. Many of the weapons were found near the bodies of men believed to be the armed guards who rounded up the Jonestown residents for the suicide ritual and forced them to stay and take the poison.

Both of the Jonestown boats also have been accounted for, Roberts said. The Marcellene, which was believed to have been missing, was another name for a fishing boat named the Cudjoe that the Guyana Defense Force found upriver from Jonestown, he said.

Heymann said that the Guyanese government had supplied U.S. authorities with fingerprint cards for most of the cult figures who entered the South American country. The fingerprint cards, plus medical and dental records will be used by technicians at Dover Air Force Base in identifying the victims.

One Justice Department official said, however, that decomposition of the bodies might make it impossible to identify positively as many as 25 percent of the 911 dead cult members at Dover.

Delaware Gov. Pierre S. DuPont IV said yesterday that he had been assured there would be no mass burial of the bodies in his state. Federal officials hope that relatives will claim the bodies and arrange for their burial individually.

Heymann noted at yesterday's briefing that the Justice Department's civil division is researching whether the government can recover the cost of transporting the bodies -- an estimated $9 million -- from the remaining assets of the Peoples Temple, which had real estate holdings in California and allegedly had large sums of money in domestic and foreign banks.

In California, meanwhile, seven elderly members of the cult were greeted by relatives yesterday on their return from Guyana.

Kay Johnson, a relative of one of the survivors, 78-year-old Carol Young, said she had attended a Peoples Temple meeting in Los Angeles just before the group left for Guyana.

"I thought the whole thing was phony, but you couldn't talk to her [Young] against Jim Jones. She had pictures of him all over the house," Johnson said.

Heymann noted at the briefing that the Justice Department has hesitated to investigate allegations of kidnaping and brainwashing of members of religious cults because of First Amendment protections. There is usually no sign of physical detention in such cases, he said, and what is brainwashing to one person may be a true conversion to another.