The suspect, 29, who was killed by police, is from Fort Pierce, Florida, the officials said.
A law enforcement source told CNN that Mateen worked as a private security guard. He rented a car and drove to Orlando to carry out the attack, the source said.
FBI teams and local law enforcement are working to clear Mateen's apartment, a federal law enforcement source said.
Agents are going through the apartment with bomb squads and investigating the materials there, the source said. Source: CNN