A man armed with an assault rifle killed 50 people at a packed gay nightclub in Orlando, Florida on Sunday in the worst mass shooting in US history, which President Barack Obama described as an act of terror and hate.
Police killed the shooter, who was identified as Omar Mateen, 29, a Florida resident and US citizen who was the son of immigrants from Afghanistan.
Mateen called 911 on Sunday morning and made comments saying he supported the Islamic State militant group, officials said.
"It has been reported that Mateen made calls to 911 this morning in which he stated his allegiance to the leader of the Islamic State," said Ronald Hopper, the FBI's assistant special agent in charge on the case.
US officials cautioned, however, that they had no conclusive evidence of any direct connection with Islamic State or any other foreign extremist group.
Fifty-three people were wounded in the rampage. It was the deadliest single US mass shooting incident, eclipsing the 2007 massacre of 32 people at Virginia Tech university.
Pulse was crowded with some 350 revelers at a Latin music night when the attack happened.
Mateen had twice been interviewed by FBI agents, in 2013 and 2014, after making comments to co-workers indicating he supported militant groups, but neither interview lead to evidence of criminal activity.
"Those interviews turned out to be inconclusive, so there was nothing to keep the investigation going forward," Hopper said.
Hopper said that Mateen was not under current investigation at the time of this incident and was not under surveillance.
Bureau of Alcohol, Tobacco and Firearms Bureau (ATF) agent Trevor Velinor said the suspect was not banned from buying firearms.
"ATF has traced those firearms. We know that this individual did purchase at least two firearms. He is not a prohibited person, so he can legally walk into a gun dealership and acquire and purchase firearms. He did so, and he did so within the last week or so. And thus far, we're following up on that, so I'm not going to get into the detail as to the specific location of the purchase. But he did purchase two firearms, a handgun and a long gun within the last few days," Velinor told reporters.
As police tried to determine what motivated Mateen's rampage, about a dozen unmarked police cars had gathered around a Port Saint Lucie house that appeared to be linked to the gunman. Police on the scene declined to comment, and neighbors said they didn't much activity in or around the white stucco home.