It is becoming increasingly unlikely that the FBI investigation of the Peoples Temple murders and mass suicides in Guyana will result in federal prosecutions on any charges, including the murder of Rep. Leo J. Ryan (D-Calif.).

Nor will officials be able to detain survivors of the cult, despite fears of former members that they might be targets of further violence.

Justice Department officials said today that their efforts have been hampered by a narrow legal jurisdiction, the inability to carry the investigation into a foreign country, and the deaths of many - if not all - of the suspects and some witnesses.

Michael Abbell, the Justice Department attorney coordinating the inquiry into Ryan's death, said that, for instance, two of the five men named by eyewitnesses as firing at Ryan's party were identified among the dead suicide led by the Rev. Jim Jones, the cult's leader.

More than 900 cult members died in the South American jungle commune of Jonestown. But since U.S. law usually does not apply in foreign countries, the investigation is restricted to seeking Ryan's murderers under a recent congressional assassination act.

Abbell said "it is likely" that all the suspects in Ryan's death are dead. But he said investigators still are checking fingerprints of the bodies flown back to the United States for positive identification.

A third suspect in the shooting of Ryan and four others, Larry Layton, has been charged and is being held in Guyana. The government there so far has refused an FBI request to send in a team of investigators. So agents have been standing by here to question the returning survivors who witnessed the murders.

Officials said today though that it is probable the Guyanese government will detain some of these witnesses for possible trials there. Seven elderly survivors from Jonestown were scheduled to arrive in New York City tonight, but officials said these survivors apparently know little about the murders at the airstrip in Port Kaituma, Guyana.

As a result of the delay, Charles Devic, the FBI special agent in charge here, said today that he has released about 20 of the 30 agents called in to help question the survivors. "We're in a holding pattern," he said. "It looks like it might be several days before anyone shows up here."

An FBI spokesman in Washington said agents across the country are checking reports that the Peoples Temple had compiled a "hit list" of prominent politicans and journalists to be kidnapped or assassinated if anything happened to Jones or the cult.

Mark Lane, an attorney for Jones who escaped the massacre at Jonestown, has said he has been questioned about such a list, but Abbell said that a list mentioned by other persons interviewed by the FBI included the name of "President Nixon."

The investigation has uncovered no evidence that this early list was credible or that there is a more current one, Abbell added.

While some of the survivors in Guyana have expressed fear and suspicion about violent behavior by others from the camp, authorities would have no basis for detaining them when they return to the United States, officials agreed.

And an FBI spokesman denied reports that new guidelines limiting investigations of domestic groups prevented the bureau from being fore-warned about the violence at Jonestown. FBI Director William H. Webster believes that imposition of the guidelines had no bearing on the outcome of the tragedy in Guyana, the spokesman said.