It is the biggest mystery in Washington at the moment. Here’s what our colleague Bobby Ghosh says his sources are telling him:

Speculation abounds about the nature of the secret program Dick Cheney asked the CIA to keep from the Congressional oversight committees. The most sensational reports suggest it was plan to find and kill top Al Qaeda leaders – like the covert Israeli campaign to take out the perpetrators of the Munich killings.

But two former ranking CIA officials have told TIME that there’s another equally plausible possibility: The program could have required the Agency to spy on Americans. Domestic surveillance is outside the CIA’s purview -– it’s usually the FBI’s job – and it’s easy to see why Cheney would have wanted to keep it from Congress.

Both officials say they were never told what was in the program, and that they’re only making calculated guesses. But their theory gibes with other reports, quoting ex-CIA officials, that say the program had to do with intelligence collection, not assassinations.

“People may want this to be about hit squads bumping off shady Saudis in Geneva, but that’s very unlikely,” says one official. “More likely, it was a plan to spy on some suspicious American citizens or organizations, without telling the FBI.”

A third CIA official who is familiar with details of the program says it was deemed unworkable and cancelled in 2004. It is not clear when or why the program was revived as a possibility, but it never got very far from the drawing board, as Republican Congressmen who received a confidential briefing about it by CIA Director Leon Panetta.