A top Syrian general under President Bashar al-Assad before his ouster has surfaced in Lebanon, where he told U.S. investigators that Austin Tice, an American journalist who went missing in 2012, was dead and provided the possible site of his grave, according to people familiar with the matter.

In meetings with the F.B.I. and the C.I.A., the former commander, Bassam Hassan, a top adviser to Mr. al-Assad who was accused of facilitating the country’s chemical weapons attacks on civilians, disclosed in recent weeks that Mr. al-Assad, the authoritarian leader who was toppled in December, had ordered the killing of Mr. Tice. The journalist was kidnapped more than a decade ago at a checkpoint in a suburb of Damascus.

The claims made by Mr. Hassan could not be independently verified, but the disappearance has long frustrated the American government.

For more than a decade, the United States has hunted for information about Mr. Tice’s whereabouts, holding out hope that he survived Syria’s brutal prison system, where torture and killings were routine. The rapid collapse of Mr. Assad’s reign after more than a half-century of family rule helped renew the search for Mr. Tice, with American investigators believing they might finally learn his fate.

Nearly a dozen people, including former and current U.S. officials, as well as past associates of Mr. Hassan familiar with the events, spoke to The New York Times on the condition of anonymity to discuss details intended to remain private or because they feared retribution.

American intelligence agencies have long believed that Mr. Hassan played a critical role in Mr. Tice’s imprisonment. His name was one of several the Biden administration gave the rebel group that toppled the Syrian government in the hopes it could obtain information about Mr. Tice. Mr. Hassan, American officials believe, could provide clues about not only Mr. Tice but also other matters of importance to the United States and its allies.