Federal authorities investigating a purported plot to kill President Carter have not decided whether they are on the track of a full-fledged conspiracy or they have heard the delusion of an intoxicated man with a history of mental illness.

Raymond Lee Harvey, a 35-year-old transient, was charged earlier this week with conspiring to kill Carter when the president was in Los Angeles last Saturday to address a Cinco de Mayo festival. Harvey had been drinking, and the Secret Service and the FBI admittedly were skeptical of his story that he was supposed to fire a starter pistol as a diversion while three other men were to shoot the president.

Since then, Harvey's tale has gained credibility from the testimony of one of the alleged plotters and the finding of a shotgun case and three rounds of ammunition in the hotel room where the conspiracy supposedly was discussed.

On Thursday, a man named by Harvey as one of the alleged conspirators-Osvaldo Espinoza Ortiz, 21-was charged with being a material witness and held on $100,000 bail. Harvey is being held on a $50,000 bond, but prosecutors asked the higher bail for Espinoza on grounds that he is an illegal Mexican immigrant who might flee.

Despite the additional evidence, both the FBI and the U.S. attorney are proceeding cautiously. Assistant U.S. Attorney Donald Etra said he would decide by next Thursday whether to present the information to a grand jury or to a preliminary hearing.

"Unless it's clear that the defendant committed the crime with which he is charged, we're not going to present the case to the grand jury," Etra said.

Etra and the FBI said the investigation is continuing.

"Any time there's a threat against a president or a possible plot against the president, we're going to take it seriously," said FBI spokesman Tom Sheil.

Harvey was arrested with a generally harmless starter pistol in his possession 50 feet from where the president spoke in the downtown Los Angeles Civic Center. He told investigators he was supposed to fire the pistol into the air and create a diversion while the president was shot.

Federal agents submitted an affidavit Thursday in which Espinoza admitted going with Harvey to the roof of the Alan Hotel the night before and firing seven rounds from the starter pistol "in order to test how loud it was."

Investigators said Espinoza also substantiated Harvey's story that two other Mexicans, armed with rifles, had lived in the hotel. They checked out of the hotel on the day of Carter's visit, and are being sought by the FBI.

One of the missing men was identified as "Umberto Camacho." His whereabouts and that of his companion are unknown.

FBI agents said Espinoza has used several aliases during his 20 months in the United States. They said they weren't sure that "Osvaldo Espinoza Ortiz" is his right name because they haven't seen his birth certificate.