Authorities here have detained a Lebanese employe of the U.S. Embassy who is suspected of having provided low-level assistance to assailants involved in the April 18 embassy bombing that killed at least 60 persons, according to a source close to the investigation.

The source said four or five other persons, including at least one Palestinian, were being held in connection with the bombing. It is unclear whether any of them have been charged, and the indications are that they are suspected of having played only a minor role.

The source indicated that Lebanese investigators, and the FBI agents who were flown here to assist them, still have no solid leads on who masterminded the blast, which killed about 60 persons, including 17 Americans. There has been widespread speculation here that Syrian and Iranian agents were involved.

Forensic evidence collected at the scene and studied by the FBI supports the original theory that the explosion was caused by a suicide terrorist driving a pickup truck who forced his way into the embassy's driveway, the source said.

He said investigators had not been able to determine the identity of the driver, adding that they found an identity card but that they think it may have been a fake one intended to mislead them.

The source said several Lebanese employes at the embassy were apprehended for questioning in connection with the bombing but only one, described as low-ranking, is still in custody. That employe is suspected of having provided information the source described as a "casing" of the embassy: the hour to attack, number of rooms in the building, roadblocks and who would likely be inside.

"It was very carefully planned, but the operation did not depend on a mole," the source said.

"The American Embassy as an institution was the target, and the bombing was timed to inflict the maximum amount of casualties," the source said.

He discounted speculation that it may have been directed at Robert C. Ames, the CIA's top Middle East analyst, who was killed in the explosion.