Where did you read or hear that?
It was reported before the FBI took over the investigation, away from the locals.
It was also reported, when the authorities detonating the bomb making material the following day to keep the area safe.
Mateen had phoned the local authorities with demands and warned them he had taken hostages and turned them into bombs, placing them at the 4 corners of the bldg so that if the police entered, he would collapse the bldg.
This ties into the Santa Monica event later in the day when that terrorist from Indiana, called local police requesting protection, because he claimed he was part of a coordinated attack with the Orlando event. Things went wrong at the last minute and he feared he was being set up by an accomplice who was bringing the rest of the bomb making material to the gay parade, but never showed.
Such is what I remember from news reports at the time.
Go Figure.