LONDON — Gerry Adams, the leader of the Irish political party Sinn Fein, was released from police custody without charges on Sunday after four days of questioning into a gruesome 1972 Irish Republican Army murder of a widow with 10 children. But the police will hand over a file of potential evidence against him to prosecutors, police officials said.

Mr. Adams, 65, turned himself in for questioning Wednesday evening and was arrested; his detention was extended a second 48 hours by a judge at the request of the police. He was released shortly before 6 p.m. local time from the police station in Antrim, Northern Ireland, escorted from a back entrance to avoid a small crowd of protesters.

The police are working on allegations made in the testimonies of Irish Republican Army dissidents, now dead, that were handed over under subpoena by Boston College, which had collected them.

Mr. Adams has been accused over the years of membership in the I.R.A. and of being the group’s commander in Belfast, Northern Ireland. In these testimonies, he was accused of having ordered the 1972 abduction, murder and secret burial of the widow, Jean McConville, who was suspected of being an informer for the British Army. Her body was found only in 2003, and the police considered hers a cold case until the testimonies emerged.