The US Federal Bureau of Investigation (FBI) had interviewed the suspect in the Orlando nightclub mass shooting three times before the fateful attack occurred on Sunday.
At least 50 people were killed and 53 more were injured in a shooting at the Pulse Club in Orlando, Florida on Sunday morning.
The gunman, identified by US media as an alleged Daesh sympathizer named Omar Mateen, a US citizen of Afghan descent from Port St. Lucie, Florida, took hostages at the club and barricaded himself in the complex but was later killed by specially trained SWAT units.
According to an intelligence official, the FBI first noticed Mateen in 2013 when he made "inflammatory comments to coworkers alleging possible terrorist ties."
The agency interviewed him twice in the course of that investigation but it was unable to verify the substance of his comments.
The FBI again carried out an investigation into possible ties between Mateen and a US suicide bomber in 2014. The agency interviewed Mateen again but determined that the contact did not constitute a threat at that time.
The FBI said it is investigating both domestic and international connections to the mass shooting that was reportedly claimed by the Daesh terrorist group.
Mateen allegedly called 911 moments before the attack, pledging allegiance to the terror group, the Lost Angeles Times reported, citing a federal law enforcement official.
Earlier on Sunday, Representative Adam Schiff, the top Democrat on a congressional intelligence committee had said that local law enforcement believed the suspect in Sunday's deadly Orlando shooting had pledged allegiance to the Daesh Takfiri terrorist group.
Suspect legally purchased firearms
An official from the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) said on Sunday afternoon that the suspect in the nightclub shooting legally purchased firearms.
ATF had traced the firearms used by the suspect and confirmed that Mateen could legally walk into a gun dealership and "acquire and purchase firearms,” said Trevor Velinor, assistant special agent in charge at ATF.
The suspect bought at least two firearms "within the last week or so," he said.