Larry Layton, a medical technician twice cuckolded by cult leader Jim Jones but loyal to him to the end, heard a federal prosecutor today describe him as the "inside" man in the tragic Guyana massacre.

Layton, a 35-year-old former Quaker, sat expressionless as opening statements began in what will be the only U.S. criminal trial to deal with the extraordinary events that took more than 900 lives on Nov. 18, 1978.

"I shot the m-----f-----," assistant U.S. attorney Robert L. Dondero quoted Layton as telling Guyanese officials who took him away from the jungle air strip where Rep. Leo J. Ryan (D-Calif.) and four others were shot to death by members of Jones' Peoples Temple. When Jones learned of the shooting, he persuaded 912 followers to join him in a mass suicide at his nearby encampment of Jonestown.

Layton's attorney, assistant public defender Frank O. Bell, immediately pointed out to the federal court jury that the evidence shows Layton did not actually kill Ryan. Instead, federal prosecutors have charged Layton with conspiracy, saying he was part of a plot by Jones to punish Ryan and others for their attempt to assist several disenchanted Jones followers in returning to the United States.

Bell said that Layton had "fallen to pieces" because of his mother's death a short time before and Jones' regular accusations that Layton was disloyal because he was hurt by Jones' affairs with both Layton's first and second wives.

Layton had no idea of the plan to attack Ryan and went to the airfield with the sole purpose of shooting the pilot of a small plane bearing four Jonestown defectors once the plane was airborne, Bell said. Layton expected the defectors inside the plane to be killed in the resulting crash, according to his defense.

The evidence in the case, as summarized by Dondero, indicates Layton posed as one of the Jonestown defectors at Jones' suggestion and accompanied the Ryan party from Jonestown back to the airfield, intending to abort their escape. After insisting that he be allowed to share the airplane with four genuine defectors, Layton allegedly shot and wounded two of them before being subdued and disarmed by a third defector.

At the same time a group of other Peoples Temple members approached a second aircraft intended to take Ryan and his party away and fired a barrage of bullets.

"While the attackers were shooting up the big plane," Dondero said, "they have done nothing to the small plane. That is an inside job." Much of Dondero's statement attempted to show that Layton was part of a preconceived plan to shoot both planeloads of Ryan's party.

Also killed that day were San Francisco Examiner photographer Greg Robinson, NBC reporter Don Harris, NBC cameraman Robert Brown and Jonestown defector Patricia Parks. But Layton has been charged only with conspiracy to murder Ryan and attempt to murder deputy chief of the U.S. mission in Guyana Richard Dwyer, because they are the only two victims covered under a federal statute governing assassination attempts against congressmen and "internationally protected persons."

Layton's life up to the shooting provides a remarkable example of Jones' hypnotic ability to inspire loyalty. Layton grew up in an affluent Berkeley family. His father was a research scientist. Layton's mother, who came from a wealthy family, divorced her husband and donated $300,000 to Jones' cause shortly before her death from cancer in 1978.

Layton joined the Peoples Temple shortly after his graduation from the University of California at Davis in sociology. He coaxed his sister, Deborah, to join the temple and remained loyal to Jones even after the temple leader had affairs with both his first and second wives. Layton's first wife, Carolyn, divorced Layton and became Jones' mistress and close adviser, acting sometimes as temple leader when Jones was not in town. She died with the other suicides at Jonestown.

Deborah, Layton's sister, who served as temple finance secretary, depositing funds in banks all over the world, eventually defected from the temple and joined those who encouraged Ryan to go to Guyana and investigate conditions there.

Layton was tried and acquitted by Guyana on charges of trying to kill the two defectors. The jury said it felt the evidence was insufficient, and some jurors appeared influenced by the fact that one of the wounded defectors would not return to Guyana to testify and the other, apparently an agnostic, refused to swear on the Bible before taking the stand.