Omar Mateen took his oath of loyalty to the Islamic State a few hours hours after he attacked the Orlando gay nightclub, surrounded by victims who were either dead, dying, or wounded, an influential lawmaker who accessed the killer's posts told FoxNews.com Wednesday.

Omar Mateen, the Afghanistan-born terrorist, killed 49 people and wounded 53 more at the Pulse nightclub in Orlando early Sunday. Mateen was eventually killed by the SWAT team that attacked the club.

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During the four hours that Mateen ransacked the club, the 29-year-old radicalized Muslim took to social media to express his hatred and swear allegiance to ISIS, Senate Homeland Security Chairman Ron Johnson (R-Wisc) said.

Mateen posted on Facebook: "I pledge my alliance to (ISIS leader) abu bakr al Baghdadi..may Allah accept me."

He also posted early Sunday morning that "the real Muslims will never accept the filthy ways of the west," criticizing the Western world for its "airstrikes" that killed "innocent women and children." He called his act "the Islamic state vengeance."

Mateen's social media accounts were shut down to prevent them viewing by the public. Johnson and his team of investigators, however, managed to see the posts.

Senate staffers sent a letter Wednesday evening to Facebook CEO Mark Zuckerberg informing him that in one of the posts, Mateen pledged loyalty to the leader of the Islamic State. Another post demanded that "America and Russia stop bombing the Islamic State."

According to Johnson, "Omar Mateen used Facebook before and during the attack to search for and post terrorism-related content."  Johnson also highlighted that his staff found out there are at least five Facebook accounts that he believed are associated with Omar Mateen.

The posts discovered by Johnson's team can explain why Mateen committed the brutal attack.

The FBI, meanwhile, has not yet issued any comment. 

TagsOmar Mateen, Senator Ron Johnson, Mark Zuckerberg, Orlando shooting