According to the Washington Post, the Americans served as neutral arbiters in the arrangement.
The exchanges, which began days after the deadly assault in late Nov, gradually helped the two sides overcome mutual suspicions and paved the way for Islamabad's announcement last week acknowledging that some of the planning for the attack had occurred on Pakistani soil, the paper quoted sources, as saying.
The intelligence went well beyond the public revelations about the 10 Mumbai terrorists, and included sophisticated communications intercepts and an array of physical evidence detailing how the gunmen and their supporters planned and executed their three-day killing spree in Mumbai.
Indian and Pakistani intelligence agencies separately shared their findings with the CIA, which relayed the details while also vetting the intelligence and filling in blanks with gleanings from its networks, the sources said.
The CIA's role was described in interviews with Pakistani officials and confirmed by US sources with detailed knowledge of the arrangement.
The arrangement is ongoing, and it is unknown whether it will continue after the Mumbai case is settled.
Officials from both countries said the unparalleled cooperation was a factor in Pakistan's decision to bring criminal charges against nine Pakistanis accused of involvement in the attack, a move that appeared to signal a thawing of tensions on the Indian subcontinent after weeks of rhetorical warfare.
A U.S. government official with detailed knowledge of the sharing arrangement said the effort ultimately enabled the Pakistani side to "deal as forthrightly as possible with the fallout from Mumbai."
"Intelligence has been a good bridge. Everyone on the American side went into this with their eyes open, aware of the history, the complexities, the tensions. But at least the two countries are talking, not shooting," the official said.
The US effort to foster cooperation was begun under the Bush administration and given new emphasis by an Obama White House, as it was feared that a renewed India-Pakistan conflict could undermine progress in Afghanistan-and possibly lead to nuclear war.&13;&13;