The U.S. government warned Saudi Arabia more than three years before the Sept. 11 attacks that Osama bin Laden might use civilian airplanes in terror attacks, according to a memo released Friday by the National Security Archive, NBC News reported.

The June 1998 note says bin Laden "might take the course of least resistance and turn to a civilian [aircraft] target."

The warning came from a U.S. regional security officer, diplomatic officials and a civil aviation official in Riyadh, Saudi Arabia. It was based on a public threat from bin Laden against "military passenger aircraft." The memo, however, said that bin Laden did "not differentiate between those dressed in military uniforms and civilians," NBC News' Robert Windrem reported.

The Sept. 11 Commission, a panel appointed by Congress in 2002 to investigate U.S. security, made no mention of the memo in any of its reports, Windrem said. It is unknown why the report did not address the warning.

The document was first disclosed by Washington Post editor Bob Woodward, in his behind-the-scenes book, "Bush at War," written in 2004. It was made public on Friday by the National Security Archive, a private group that uses Freedom of Information requests to obtain classified data.

On Friday, the National Security Archive also released a letter from former CIA Director George Tenet written five days after Sept. 11. Titled "We're at War," the letter to top deputies urges an "unrelenting focus" on using all of America's capabilities, "not only to protect the U.S. both here and abroad from additional terrorist attacks — but also, and more importantly, to neutralize and destroy al-Qaida and its partners."

The Sept. 16 letter was written in the wake of criticism directed at Tenet and the CIA for the agency's shortcomings in preventing an assault on U.S. soil.

There have been a slew of reports over the past decade of plots to use planes to strike American targets. In 1995, U.S. and Filipino authorities uncovered a plot by Ramsey Yusef, nephew of Khalid Sheikh Mohammed, the mastermind behind Sept. 11. Yusef threatened to hide bombs on planes and blow them up over the Pacific.

The most notable security warning, Windrem said, was a presidential briefing on an Aug. 6, 2000, that mentioned the possibility of passenger airliners being used in terrorist attacks.