A 33-year cover-up by six administrations has been unwrapped with the recent declassification of a State Department document fingering Yasser Arafat in the murder of three diplomats, two Americans and a Belgian, in Sudan in 1973.

They were machine-gunned with Arafat’s “full knowledge and personal approval,” according to the once “secret” memo.

The revelation was less of a surprise than a confirmation of long-held suspicions and leaves many questions that demand answers. At a time when fighting terrorism tops the American security agenda, this should be a high priority for the 110th Congress that convenes this week.

The House International Relations and Senate Foreign Relations committees should investigate and hold public hearings on why every administration since Richard Nixon, Republican and Democrat alike, covered up for Arafat and elevated him from terrorist to would-be peacemaker.

It’s too late to arrest and try Arafat, who died Nov. 11, 2004, but that doesn’t mitigate the need for answers.

On March 1, 1973, eight Black September terrorists seized the Saudi Embassy in Khartoum during a going-away party for charge d’affaires Curtis Moore. The group, an arm of the PLO and Fatah, both of which Arafat headed, demanded the release of Black September leader Abu Daud, the mastermind of the Munich massacre of Israeli Olympic athletes six months earlier, and others held by Jordan, Israel, and the United States.

But Arafat’s real motivation, according to the recently declassified State Department document, was to “strike at the United States because of its efforts to achieve a Middle East peace settlement which many Arabs believed would be inimical to Palestinian interests.”

The memo clearly states, “The Khartoum operation was planned and carried out with the full knowledge and personal approval of Yasir Arafat.”

Other published reports indicated U.S. intelligence had advance warning but failed to act quickly enough to prevent the takeover. Twelve hours after the hostage-taking began, Moore, U.S. Ambassador Cleo Noel, and Belgian charge d’affaires Guy Eid were murdered.

The State Department document also further confirms Arafat’s linkage to the Munich massacres. It identifies Black September as part of the PLO and operating on his orders in Khartoum, with a stated goal of freeing the Munich mastermind, Abu Daud. In his 1999 memoir, Daud took responsibility for Munich and said it had been planned with Arafat’s explicit knowledge.

Rumors of his involvement reached me more than 20 years ago during Ronald Reagan’s first term, when I was legislative director of the American Israel Public Affairs Committee. We’d been told that the CIA had an audiotape of Arafat giving the order to kill Noel, Moore, and Eid.

I asked Sen. Arlen Specter (R-Pa.), a member of the Senate Select Committee on Intelligence and the Appropriations Committee with jurisdiction over intelligence spending, to look into the matter. He reported back that he personally went to then-CIA director William Casey and was told no such tape existed.

Subsequently, we found out that a more appropriate address was the National Security Agency, the ones in charge of electronic eavesdropping. Post-9/11 we all learned that our intelligence agencies do a poor job of communicating with each other. Could that have been the answer?

There are more questions lawmakers should be asking. Why the cover-up? Was the NSA more interested in protecting its “sources and methods” than in catching the killers? Or did they just pass the buck, and to whom?

Who else knew about this and what did they do about it?

Was Casey, not a master of veracity, leveling with Specter or part of the cover-up?

Was the CIA, which had back-channel links to Arafat in Beirut, protecting its own sources and operations?

Was the Nixon White House, already consumed with Watergate, intent on avoiding another scandal by concealing news that it had bungled an attempt to prevent the deaths of the three diplomats?

Did the State Department want to use the information privately to motivate Arafat to change his ways and join a peace process?

Would the exposure of Arafat’s role have helped force a change to more moderate PLO leadership?

One thing I know is we asked the wrong question and accepted Casey’s answer too easily.

We should have been inquiring more broadly, to paraphrase Sen. Howard Baker’s famous question of the era, about what the U.S. government knew about the Khartoum killings and when they knew it. And we at AIPAC should have pressed our friends in the Congress to try to find out.

There are those who will point to the memo as more proof that it was wrong from the get-go to deal with Arafat, who was a terrorist until the day he died. No one ever believed Arafat had clean hands — except maybe Jimmy Carter — but successive American and Israeli administrations recognized that nothing could be achieved by excluding him and if anything could be accomplished they had to deal with him.

The State Department has provided the “smoking memo” clearly stating Arafat’s direct involvement and exposing the 33-year cover-up.

It’s time for Congress to begin asking the tough questions, starting with, Why cover up for Arafat?