For decades, Israel has been trying to recover the remains of Eli Cohen, one of its most famous spies, who was executed in Syria in 1965.

While that goal remains elusive, intelligence services and the prime minister’s office on Sunday implied that they may have gotten one step closer by acquiring documents and personal affects from Syria that belonged to Mr. Cohen.

The trove of 2,500 items includes documents and photographs from Mr. Cohen’s years undercover, information about his final moments, personal artifacts taken from his home and handwritten letters to family members, Prime Minister Benjamin Netanyahu’s office said in a statement.

The Mossad, Israel’s foreign intelligence service, worked together with an allied foreign government to retrieve the archives, the prime minister’s office said. It did not elaborate on which country helped and when exactly it had recovered the documents. It was not clear how the government acquired the documents.

During his three years as an undercover agent in Syria in the early 1960s, Mr. Cohen fostered close relationships with top Syrian officials and provided substantial information to Israel, including about Syria’s military, its relationship with the Soviet Union and power struggles within the leadership. Syria was Israel’s main rival in the region at that time.

Two years after his death, Mr. Cohen’s information helped Israel achieve victory in the Arab-Israeli war of 1967, also known as the Six-Day War, and seize the Golan Heights from Syria.