Orlando shooting: What motivated a killer?

Updated 8:24 AM EDT, Tue June 14, 2016
ORLANDO, FLORIDA - JUNE 12:  FBI agents seen outside of Pulse nightclub after a fatal shooting and hostage situation on June 12, 2016 in Orlando, Florida. The suspected shooter, Omar Mateen, was shot and killed by police. 50 people are reported dead and 53 were injured. (Photo by Gerardo Mora/Getty Images)
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ORLANDO, FLORIDA - JUNE 12: FBI agents seen outside of Pulse nightclub after a fatal shooting and hostage situation on June 12, 2016 in Orlando, Florida. The suspected shooter, Omar Mateen, was shot and killed by police. 50 people are reported dead and 53 were injured. (Photo by Gerardo Mora/Getty Images)
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Seddique Mateen, the father of Omar Mateen, who attacked a gay night club in Orlando before being shot dead by police, walks into his home in Port Saint Lucie, Florida, U.S. June 14, 2016. REUTERS/Carlo Allegri (Newscom TagID: rtrlseven934325.jpg) [Photo via Newscom]
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Seddique Mateen, the father of Omar Mateen, who attacked a gay night club in Orlando before being shot dead by police, walks into his home in Port Saint Lucie, Florida, U.S. June 14, 2016. REUTERS/Carlo Allegri (Newscom TagID: rtrlseven934325.jpg) [Photo via Newscom]
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(CNN) —  

While the bodies of Omar Mateen’s victims were removed from Pulse nightclub overnight, investigators worked Monday to determine what motivated the Florida killer responsible for the worst mass shooting in U.S. history – a man the FBI knew was sympathetic to terrorism and had previously investigated.

The attack appears to be “homegrown terrorism” carried out by legally purchased firearms, President Barack Obama said in the Oval Office. He said there wasn’t any evidence the attacker was under direction from a terrorist network, or carrying out any group’s larger plot.

In Washington, FBI Director James Comey defended the agency’s investigation of Mateen, a 29-year-old security guard from Fort Pierce, Florida, who was placed on a terror watch list and was looked into twice. Agents found no reason to think he was a credible threat, Comey said.

Investigators are “highly confident” the gunman was self-radicalized through the internet, Comey said.

According to one official, analysis of Mateen’s electronic devices showed searches for jihadist propaganda, including videos of ISIS beheading videos and of Anwar al-Awlaki – an influential American-born imam who worked as a spokesman for al Qaeda in the Arabian Peninsula and was killed in 2011.

“He consumed a hell of a lot of jihadist propaganda” online, the source said.

The presumptive presidential nominees weighed in. Democrat Hillary Clinton called for a ban on assault weapons and Republican Donald Trump once again said the United States needs to ban Muslims from entering the country.