On the same day as the Orlando attack, an Indiana man armed with three assault rifles and chemicals used to make explosives was arrested in Southern California and told police he was headed to a West Hollywood gay pride parade, authorities said.

The previous deadliest mass shooting in modern U.S. history was the 2007 attack at Virginia Tech, where a student killed 32 people and took his own life.

Mateen was not unknown to law enforcement: In 2013, he made inflammatory comments to co-workers and was interviewed twice, according to FBI agent Ronald Hopper, who called the interviews inconclusive. In 2014, Hopper said, officials found that Mateen had ties to an American suicide bomber, but the agent described the contact as minimal, saying it did not constitute a threat at the time.

Mateen bought at least two guns legally within the last week or so, according to Trevor Velinor of the Bureau of Alcohol, Tobacco and Firearms.

Mateen exchanged gunfire with 14 police officers at the club and took hostages at one point. In addition to the assault rifle, he had a handgun. Police Chief John Mina said officers held back for some time because Mateen indicated he had a bomb vest.

About 5 a.m., authorities sent in a SWAT team to rescue the remaining club-goers, Mina said.

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Associated Press writers Eric Tucker in Washington, Terrance Harris, Mike Schneider and Tamara Lush in Orlando and photographer Alan Diaz in Fort Pierce, Florida, contributed to this report.