People mourn the victims of the mass shooting during a vigil at a park in Orlando, the United States, on June 12, 2016. At least 50 people were killed and 53 others wounded early Sunday in a shooting spree at a gay nightclub in Orlando, Florida. [Photo: Xinhua/Yin Bogu]

US investigators say they have uncovered strong indications the gunman in the deadly Orlando shooting had become radicalized and potentially inspired by foreign terrorist groups.

However, FBI Director James Comey says their preliminary probe indicates there was no direct connection or contact between 29-year old Omar Mateen and forces outside the United States.

"There are strong indications of radicalization by this killer and of potential inspiration by foreign terrorist organizations. We are spending a tremendous amount of time as you would imagine trying to understand every moment of this killer's path to that terrible night in Orlando. To understand his motives and to understand the details of his life. So far, we see no indication that this was a plot directed from outside the United States, and we see no indication that he was part of any kind of network."

Comey has also confirmed the FBI spent 10 months investigating Mateen in 2013 and 2014.

The probe into Mateen came after he claimed to co-workers that he had "family connections to al-Qaida" and made statements that raised concerns about possible ties to terrorism.

Authorities closed their probe after concluding that Mateen did not have ties with terror groups.

The FBI director has also revealed new details about Mateen's conversations with a 911 dispatcher early Sunday morning as he was carrying out the attack.

During the course of three phone calls, Mateen pledged allegiance to the head of the Islamic State.

He also claimed solidarity to the Boston Marathon bombers and to an American suicide bomber who blew himself up in Syria in 2014.

However, media interviews with survivors and regulars to the nightclub are suggesting Mateen went to the club several times before the incident, and may have - himself - been gay, suggesting he may have been struggling with his own identity.

Meanwhile, the names of all 49 victims have been released.

Most of the victims are hispanic, as the Orlando nightclub was holding its Latino night when the attack took place.

Most of the 53 people wounded in the attack remain in hospital.

Five of them are still listed in critical condition.