Washington, May 9 (IANS) US President Barack Obama gave the go-ahead for a raid on the Al Qaeda founder's hideout in Pakistan on "what was probably a 50-50 chance that Osama bin Laden was there", his national security adviser said.

"It was a circumstantial case ... But what he had 100 percent confidence in was the ability of our special forces to execute the mission," Tom Donilon told CNN in an interview aired Sunday.

Obama attended April 28 the last of several National Security Council meetings focused on finding and going after the Al Qaeda leader.

During that meeting, some advocated for the commando raid while others advised against it, Donilon said, given there had been no clear-cut sightings of bin Laden by that point.

"He had gotten divided counsel, and that happens a lot in these things, as you would imagine," Donilon said.

After a night's sleep, Obama told Donilon at 8.20 a.m. the next day to draft the order for the raid.

By Sunday evening - which was early Monday morning in Pakistan - the 38-minute mission was over, the 25-strong US team having flown out of the country along with bin Laden's body.

Bin Laden's death is a major blow to Al Qaeda, said Donilon, describing it as "the single biggest achievement we've ever had" in the fight against Al Qaeda.

"At the end of last year, we assessed that Al Qaeda had been diminished ... to its weakest since 2001. And they took a very big step back Sunday," he said.

One reason for that assessment is what Donilon called the "extraordinary" amount of intelligence that US special forces were able to take from the Abbottabad compound.

"This is the largest cache of information gotten from a senior terrorist -- gotten from any terrorist in one operation," Donilon said, calling the amount of seized material about equal to "the size of a small college library."