The CIA maintained a safe house in the Pakistani city of Abbottabad for a small team of spies who conducted extensive surveillance over a period of months on the compound where Osama bin Laden was killed by U.S. Special Operations forces this week, according to a report in the Washington Post.

The secret CIA facility was used as a base of operations for one of the most delicate human intelligence gathering missions in recent CIA history, one that relied on Pakistani informants and other sources to help assemble a pattern of life portrait of the occupants and daily activities at the fortified compound where bin Laden was found, the report quoted an official as saying.

The effort was so extensive and costly that the CIA went to Congress in December to secure authority to reallocate tens of millions of dollars within assorted agency budgets to fund it, the report said quoting another U.S. official.

The CIA's job was to find and fix, said a U.S. official, using Special Operations forces terminology for the identification and location of a high-value target. The intelligence work was as complete as it was going to be, and it was the military's turn to finish the target.

Most of that surveillance capability remained in place until the execution of the raid by U.S. Navy SEALs shortly after 1 a.m. in Pakistan, the report added.