Orlando police entered the apartment of Omar Mateen, the suspected killer in Sunday's terrorist attack on Pulse nightclub, after finding the door left wide open on Monday.

A CBS News reporter said the door must have been open for "the better part of two hours."

Police head into #Orlando shooter's apartment after door found open https://t.co/0xX3vxIi6f https://t.co/oA0I78oEoK

— CBS News (@CBSNews) June 13, 2016

Police are reportedly investigating the incident as a possible burglary.

The Fort Pierce apartment where Orlando nightclub shooter Omar Mateen last lived. pic.twitter.com/vjFteVYUz6

— Kevin Deutsch (@NewsdayCrime) June 13, 2016

The news comes amid an FBI investigation into Mateen’s possible ties to Islamic terror groups. Any tampering with items left in the suspect’s apartment could prove detrimental to this pursuit.

#FBI finishes search of #Orlando shooter's condo.https://t.co/Jdu8rIWAA9 pic.twitter.com/uq3Xjf0gNh

— KUTV 2News (@KUTV2News) June 13, 2016

In an interview with KUTV-TV, one of Mateen's neighbors described the 29-year-old as noticeably quiet compared to other residents living in the Fort Pierce condominium complex.

"Everybody around here says, 'Good morning,' or 'How are you doing' — just, you know, common talk — and he [Mateen] never said anything," the neighbor said.

This is a breaking news story and will be updated.