BEIRUT, Lebanon (AP) _ A Lebanese court on Thursday acquitted one man and convicted another of complicity in the 1976 kidnapping and slaying of a U.S. ambassador, an American aide and their driver.

But the court said the killer, believed to be an agent of a Palestinian guerrilla group, remains at large.

The three-man tribunal convicted Nameq Ahmed Kamal, 44, of aiding in the slaying and sentenced him to four years in prison. The court cited a lack of evidence in handing down the relatively light sentence.

Bassem Mohammed Farkh, 37, was acquitted of charges in the death of Ambassador Francis E. Meloy, his American economic counselor, Robert O. Waring, and his Lebanese driver, Mohammed Moghrabi.

On May 5, Kamal and Farkh were convicted in absentia by the same court and sentenced to life in prison with hard labor. Kamal later surrendered to police and Farkh turned up in jail on another charge.

Under the law, a new trial is required when a suspect convicted in absentia turns up and objects to the verdict and sentence.

Kamal and Farkh pleaded innocent during the two-month trial. But they admitted they were present when Meloy and Waring were kidnapped at a checkpoint on the Green Line that split Beirut into Christian and Muslim sectors during Lebanon's 1975-1990 civil war.

The two Americans were crossing in their chauffeured embassy car from the Muslim sector to the Christian side in June 1976 when they disappeared at a checkpoint manned by the Popular Front for the Liberation of Palestine.

Their bullet-riddled bodies were found several days later on a beach in Muslim west Beirut.

The PFLP is a Marxist guerrilla faction headed by George Habash with headquarters in Damascus, Syria. It opposes the PLO-Israel accord on Palestinian autonomy in the Gaza Strip and Jericho.

The court's verdict held a Palestinian PFLP officer who went by the nom de guerre of Mahmoud Awadah responsible for the slayings.

But the court said legal proceedings could not be initiated against Awadah because his true identity is not known.