Orlando gunman Omar Mateen apparently made a series of Facebook posts before and during his attack on a gay nightclub in Orlando, raging against the "filthy ways of the West" and blaming the US for the deaths of "innocent women and children".
The killer, whose rampage left 49 people dead, also searched for "Pulse Orlando" and "Shooting" online on the morning of the carnage and said on Facebook: "America and Russia stop bombing the Islamic state," according to a Senate committee letter.
The messages were detailed in a letter from the Homeland Security Committee to Facebook chief executive Mark Zuckerberg asking the company to produce information on Mateen's online activity and to provide a briefing to the panel.
The letter was released as the procession of memorials and funerals for the victims began in Orlando and the FBI appealed for the public's help in reconstructing the killer's movements.
Investigators also zeroed in what role Mateen's wife might have played in the attack at Pulse dance club.
"We need your help in developing the most complete picture of what he did and why he did it," FBI agent Ron Hopper said at a news conference.
In its letter, the committee said staffers have learned that five Facebook accounts were associated with Mateen, a 29-year-old American-born Muslim.
"The real muslims will never accept the filthy way of the west," Mateen wrote, according to the letter.
Mateen pledged his allegiance on Facebook to the leader of Islamic State and, in his final post, warned: "in the next few days you will see attacks from the Islamic state in the usa."
Despite the professed loyalty to the extremist group, the Obama administration said it has seen no evidence that the shooting rampage was directed by IS.
The three-hour rampage began at 2am on Sunday and ended with Mateen being killed by a police SWAT team.
The FBI said it is still gathering evidence at Pulse and analysing mobile phone location data to piece together Mateen's activities leading up to the massacre, while also interviewing people who had any dealings with him.
On Saturday night, hours before the rampage, Mateen visited Disney Springs, an outdoor restaurant, retail and entertainment complex at Walt Disney World, an official who was briefed on the case said.
The official said it is not clear what Mateen was up to.
A key topic for investigators is how much Mateen's Palestinian-American wife might have known about the plot.
An official said authorities believe 30-year-old US-born Noor Salman knew ahead of time about the attack.
Investigators have spoken extensively with her and are working to establish whether she recently accompanied Mateen to the club, said a second official.
Prosecutor Lee Bentley repeatedly refused to say whether charges might be brought against the wife or anyone else. He said authorities are talking to hundreds of people and investigating everyone associated with Mateen, including family, friends and business associates.
Salman has been in seclusion for days.