Orlando shooter posted on Facebook and searched for news on attack as he gunned down victims

An undated photo from a social media account of Omar Mateen.
An undated photo from a social media account of Omar Mateen.PHOTO: REUTERS

ORLANDO (AFP) - A top United States lawmaker said on Thursday (June 16) that the authorities were probing the Facebook activity of the Orlando nightclub shooter, after an investigation found Omar Mateen made extremist posts during the massacre and searched for news of his attack.

Mr Ron Johnson, the Republican chairman of the Senate Committee on Homeland Security, sent a letter to Facebook chief Mark Zuckerberg on Wednesday asking the company to share specifics of five accounts apparently used by Mateen.

Lawmakers are trying to determine if there are ways for intelligence and law enforcement communities to monitor social media platforms like Facebook "so that we can prevent these tragedies", Mr Johnson told CNN on Thursday.

"This is our job, to see what has happened in the past, what can we possibly do to prevent this from occurring in the future and how can we find bipartisan solutions," the Republican said.

Mateen killed 49 people and wounded 53 in his shooting spree early Sunday at the Pulse gay nightclub in the Florida resort city of Orlando. The authorities have said he was apparently radicalised after watching extremist propaganda online.

In his letter to Mr Zuckerberg, Johnson said Mateen apparently posted sometime during the attack that he was pledging allegiance to the Islamic State in Iraq and Syria (ISIS) leader, and "America and Russia stop bombing the Islamic state".

He also allegedly posted "The real muslims will never accept the filthy ways of the west" and "In the next few days you will see attacks from the Islamic state in the usa".

Mateen also allegedly searched for "Pulse Orlando" and "Shooting" as his victims lay dead or dying, apparently seeking to gauge reaction in real time.

A Facebook spokesman confirmed the company has received the senator's letter, which asked Facebook to provide investigators with details of Mateen's Facebook accounts and activity.

She said Facebook had not yet responded to the request but has been working with law enforcement from the outset of the investigation.

Also on Thursday, CNN reported that Mateen's wife suspected he was going to carry out an attack that night, even though he told her he was going out to see a friend.

When the news of the shooting broke, 30-year-old Noor Mateen called him frantically, CNN said, citing unidentified sources. He didn't pick up, but texted her about 4am asking if she had seen the news.

The report said she responded: "I love you."

The authorities have refused to comment on reports that Mateen's wife would face charges over her alleged knowledge of Mateen's intentions to carry out an attack.