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U.S. Senate Homeland Security Chairman Ron Johnson (R-Wis.) is asking Facebook to turn over material from five accounts used by Omar Mateen before and during his assault Sunday on a gay night club in Orlando, Fla.

In his Wednesday letter to Facebook Chairman Mark Zuckerberg, Johnson said his staff has learned that Mateen was searching out "Pulse Orlando" and "shooting" on Sunday.

"It is my understanding that Omar Mateen used Facebook before and during the attack to search for and post terrorism-related content," Johnson wrote in the two-page letter.

Mateen killed 49 people and injured dozens of others before he was killed by police. It was the worst mass shooting in American history.

On Sunday, Mateen "apparently posted 'America and Russa stop bombing the Islamic state,'" and also pledged his "alliance" with Islamic State leader Abu Bakr al-Baghdadi, Johnson wrote.

"He then posted 'The real muslims will never accept the filthy ways of the west' and 'You kill innocent women and children by doing us airstrikes ... now taste the Islamic state vengeance,'" according to Johnson's letter.

"In a final post, Mateen apparently wrote, 'In the next few days you will see attacks from the Islamic state in the usa,'" Johnson wrote.

In the letter, Johnson said his staff learned that in May, Mateen used Facebook to search for information on the couple behind the December terror attack in San Bernardino, Calif.

On June 4, Mateen "apparently searched 'Baghdadi Speech,'" Johnson said. Johnson added that his staff understood that Mateen "apparently used Facebook to conduct frequent local law enforcement and FBI searches, including searching for specific law enforcement officers."

The first-term Wisconsin senator gave Zuckerberg until June 29 to turn over to his Senate committee all activity logs, timeline information, messages, photos and posts related to the five Facebook accounts associated with Mateen.

In a separate letter to FBI Director James Comey, Johnson said his committee is conducting an inquiry "to better understand the root causes of homegrown extremism, assess the vulnerabilities of so-called 'soft' targets, and oversee the work of federal law-enforcement entities."

Johnson asked for information about actions taken by the FBI "before, during and after the attack."

He asked the FBI about what it knew regarding Mateen's Facebook activity. In addition, Johnson's letter raised specific questions about Mateen's activities. For instance, he asked about all details about an international flight Mateen took in March 2012, and a "device" he was wearing when his body was found in the nightclub. He also asked whether Mateen, his wife or his father were under FBI surveillance in the days prior to the attack or were on a terrorism watch list.

Johnson also asked whether the FBI was contacted by Mateen's employer or co-workers.

Comey was also given until June 29 to produce the requested information.