"I had real reservations about the intelligence," Gates, who earlier led the Central Intelligence Agency, said. "But it was the best information we had since probably 2001."
Gates, speaking to CBS's "60 Minutes" in a segment airing Sunday night, said he worried about the risk to American lives had the assault gone bad.
He said his biggest worry now is a terrorist gaining control of a weapon of mass destruction, especially in this country, because for years the United States has received intelligence that terrorists are trying to acquire such weapons.
Gates, asked whether Bin Laden’s killing meant troops could withdraw from Afghanistan on an accelerated basis this summer, called that decision premature.
He said, though, that if the U.S. kept up military pressure there, circumstances might allow a reconciliation process to advance this fall or winter between the Afghanistan government and the Taliban.
Gates, who steps down next month, was appointed defense chief by President George W. Bush after midterm congressional elections in 2006. He is the only defense chief in history asked to remain on the job by a newly elected president. Gates has served eight presidents in all.
He said signing condolence letters for fallen troops was not a task he took lightly.
"I swore I would never let any of them become a statistic for me," he said.
Gates said with every fatality, he asked for a photo and a packet of hometown news accounts so he could read what coaches, parents, brothers and sisters had to say about the fallen. That way he felt he knew them.
"In some ways, it makes the job harder," he said. "But I want the parents or the wives or spouses to know that I care about every single one of them."