Declassified documents from the 1970s provide new evidence that federal officials believed bomb-grade uranium that disappeared from a Pennsylvania nuclear facility in the 1960s was likely taken for use in a clandestine Israeli atomic-weapons program.

The documents, obtained earlier this year through public-records requests by a Washington-based nonprofit group, also indicate that senior officials wanted to keep the matter under wraps for fear it could undermine U.S. Middle East peace efforts.

Though the Central Intelligence Agency's case for the suspected theft wasn't conclusive, it was sufficiently persuasive that "I do not think that the President has plausible deniability" regarding the question, said a memo dated July 28, 1977, by a National Security Council staffer in President Jimmy Carter's administration.

A security council memo to Mr. Carter a few days later expressed more uncertainty about whether a theft had occurred, but noted that then-Secretary of State Cyrus Vance had a coming Middle East trip and discussed the need to keep attention "away from the CIA's information."

The question of whether one of America's closest allies was involved in the theft of some of its most valuable and dangerous material in pursuit of nuclear weapons has been one of the enduring mysteries of the atomic age. The suspected theft has drawn the attention of at least three presidents and other senior government officials.