U.S. authorities were due on Monday to release partial transcripts of 911 calls made during last week's mass shooting by a gunman who slaughtered 49 people at a gay nightclub in Orlando, Florida, before being killed by police.

Omar Mateen, 29, is said to have paused during a three-hour siege to telephone emergency dispatchers three times and to post internet messages from inside the Pulse nightclub professing his support for Islamist militant groups.

The FBI was due to hold a news conference near the club at 11 a.m. EDT (1500 GMT) to provide an update on the investigation and to release the partial transcripts of the 911 calls.

Attorney General Loretta Lynch said they would include the "substance of his conversations" recorded as Mateen carried out the deadliest mass shooting in modern U.S. history, but not any pledge of loyalty he is alleged to have made to the Islamic State militant group.

Authorities have said preliminary evidence indicates Mateen, who worked as a security guard, was a mentally disturbed individual who acted alone and without direction from outside networks.

Lynch, who is due to visit Orlando on Tuesday, told CNN on Sunday that investigators have been focused on building a full profile of Mateen, a New York-born U.S. citizen and Florida resident of Afghan descent, who has been described by U.S. officials as "self-radicalized" in his extremist sympathies.

The Pulse massacre, which also left 53 people wounded, led to a week of national mourning and soul-searching over access to firearms and the vulnerability to hate crimes of people in the gay, lesbian, bisexual and transgender community.

While in Orlando, Lynch will meet with investigators, as well as survivors and loved ones of the victims.

The massacre has triggered an effort to break a long-standing stalemate in Congress over gun control.

The Senate was set to vote on Monday on four competing measures - two from Democrats and two from Republicans - to expand background checks on gun buyers and curb gun sales for people on terrorism watch lists.

Donald Trump, the presumptive Republican nominee for the Nov. 8 presidential election, has said he shares the goal of keeping guns out of the hands of people on watch lists.

Trump said on Monday he was referring to security staff, not patrons, when he said that if more people had been armed in the nightclub, fewer would have died.