Thomson Reuters Police cars and fire trucks outside the Pulse night club in Orlando
The threat prompted the Orlando Police Department to storm Pulse nightclub. Mateen was shot and killed after shooting at the entering officers.
During the course of the shootout and subsequent standoff, Mateen told police negotiators that he planned to put explosive vests on four hostages and place them in corners of the nightclub, Dyer said.
"We had independent verification of that. We had a lot of information from the inside and they independently were saying, 'Yes, the bomber is about to put on an explosive vest,'" Dyer said.
In a press conference Monday, Orlando police said they spoke with Mateen at least three times over the course of several hours on Sunday morning. Orlando Police Chief John Mina said Mateen appeared "cool and calm" throughout the attack.
"He really wasn't asking for a whole lot. We were doing most of the asking," Mina said. "Our negotiators were talking with him, and there were no shots at that time, but there was talk about bomb vests and explosives. There was an allegiance to the Islamic State."
Mina said police decided to storm Pulse because they believed that they were facing an "imminent loss of life."
After Mateen was killed, police were worried that the building might be "booby-trapped" with explosives, a worry that was initially reinforced by the crime scene, Dyer told reporters at the Wednesday press conference.
"When the shooter was killed, you could see a battery pack right next to him, which would indicate to us that there's a detonator of some sort. There was also a bag near his body, so you would logically lead to the conclusion that the bag contained explosives and he had some type of detonator that could have been a pressure detonator that was under the body," Dyer said.
It took several hours to clear the nightclub to make sure it wasn't rigged with explosives.
Mateen was living in Fort Pierce, Florida, when he rented a car and drove to Orlando to carry out the attack on Sunday morning. He was armed with an assault-style rifle and a handgun that he had legally bought days before. He was a security guard and had a Florida firearms license that allowed him to carry concealed weapons.
He called 911 and a local TV news station during his rampage at Pulse and pledged allegiance to the Islamic State, law-enforcement officials have said. In his call to 911, he also expressed sympathy for the Boston Marathon bombers and an American suicide bomber who was aligned with Al Qaeda in Syria. He was investigated by the FBI in 2013 and 2014 for suspected ties to terrorists.
The attack was the deadliest mass shooting in US history.