Omar Mateen, the Afghan-origin gunman who killed at least 50 people at a gay nightclub in Orlando in US' deadliest mass shooting incident, pledged allegiance to ISIS in a phone call to emergency number 911 during the attack.
Mateen, a 29-year-old security guard who was born in New York to Afghan parents, called 911 after his initial assault on the dance club to pledge allegiance to the leader of the Islamic State Abu Bakr al-Baghdadi, law enforcement officials were quoted as saying by media reports.
During the call, he also mentioned the Boston bombers -- Tsarnaev brothers, who carried out the Boston Marathon bombings in 2013.
Mateen, who carried an assault rifle and a pistol into the gay club in Orlando in the wee hours of Sunday and killed 50 people while injuring 53 others, was later shot dead by the police.
He made the 911 call from a bathroom of the gay club in Orlando, Florida. During the call he gave his full name, officials said.
US President Barack Obama has described the massacre as an act of terror and an act of hate. The FBI is investigating the case as an act of terrorism.
"It appears he was organised and well-prepared," Orlando Police Chief John Mina told media persons.
The police said they have not found any accomplices so far. According to the police, Mateen was previously under FBI radar. FBI had interviewed him in 2013 and 2014. But at that time the FBI did not find anything against him.
"Those interviews turned out to be inconclusive, so there was nothing to keep the investigation going," FBI Assistant Special Agent Ronald Hopper was quoted as saying.
Since 2007, Mateen had worked as a security officer at G4S Secure Solutions, which is said to be one of the world's largest private security companies.
G4S, in a statement, said they are "shocked and saddened by the tragic event" and are cooperating with law enforcement agencies.
According to Anti-Terrorism Task Force Assistant Special Agent in Charge Trevor Velinor Mateen legally purchased a Glock pistol and a long gun in the last two weeks.
Officials are still trying to find out the motives behind the attack.
While ISIS has taken credit for the attack, the FBI is yet to find evidence that corroborates this claim.
Some terrorism experts described this as the largest act of terrorism after 9/11 attacks.
Officials and experts fear that there might be similar attacks inside the country in particular from lone home grown radicalised terrorists in the months to come.