Radical Islamic terrorist Omar Mateen checked Facebook in the midst of his vicious slaughter of 49 innocents at a gay nightclub in Orlando, Florida to see if he was trending.

According to a letter from a Senate committee to Facebook founder Mark Zuckerberg, the terrorist searched “Pulse Orlando” and “shooting” during his attack, which began at 2 a.m. on Sunday morning, reports The New York Daily News.

Mateen, 29, pledged his allegiance to the Islam State via a 911 call during the massacre. The terror group later claimed credit for the attack via an affiliated "news agency."

He also posted chilling messages to the social media network early Sunday morning:

“America and Russia stop bombing the Islamic state..I pledge my alliance to abu bakr al Baghdadi..may Allah accept me,” posted the killer on Facebook, explicitly citing his allegiance to the ISIS leader.

“The real muslims will never accept the filthy ways of the west,” he wrote in another post. “You kill innocent women and children by doing us airstrikes..now taste the Islamic state vengeance.”

The murderer’s final post warned of more attacks. “In the next few days you will see attacks from the Islamic state in the usa,” he wrote.

Mateen's search history tied him further to radical Islamic. In May, Mateen looked up information regarding the radical Islamic terrorists from San Bernardino, California, Syed Farook and Tashfeen Malik. Their horrific attack left 14 people dead.

“On June 4 he looked up 'Baghdadi Speech,'” notes the NYD News.

“My staff has also learned that Mateen apparently used Facebook to conduct frequent local law enforcement and FBI searches, including searching for specific law enforcement offices,” wrote Republican Senator Ron Johnson, chairman of the senate committee.

Despite his reported online activity and his red flag-raising history, Mateen was not on a terror watch list at the time of the attack. He was briefly investigated twice before, but eventually cleared for lack of incriminating evidence.

“Mateen became a person of interest in 2013 and again in 2014,” reports The Daily Beast. “The Federal Bureau of Investigation at one point opened an investigation into Mateen, but subsequently closed the case when it produced nothing that appeared to warrant further investigation.”

Mateen was once investigated for “suggesting an affinity for Islamist groups, and a second time for vague connections to another Florida man who traveled to Syria to become a suicide bomber,” reports The Washington Post.

A law-enforcement source via The Daily Beast said Mateen was “a known quantity.” He added, “He’s been on the radar before.”

Despite the suspicion and the FBI investigation, since 2007 Mateen had been a security guard for G4S, "a large multinational security company that provides services to more than two-dozen juvenile-detention facilities in Florida.”

His barbaric rampage at the gay nightclub Pulse left 49 dead and 53 injured, 6 critically.