The wife of the gunman who killed 49 people at an Orlando gay nightclub knew of his plans for the attack and could soon be charged in connection with the deadliest mass shooting in modern US history, a law enforcement source said.

The source told Reuters that a federal grand jury had been convened and could charge Omar Mateen's wife, Noor Salman, as early as Wednesday.

"It appears she had some knowledge of what was going on," said US Senator Angus King, a member of the Senate Intelligence Committee, which received a briefing on the attack on Tuesday.

"She definitely is, I guess you would say, a person of interest right now and appears to be cooperating and can provide us with some important information," King told CNN.

Mateen, who was shot dead by police after a three-hour standoff at the Pulse club early on Sunday, called 911 during his shooting spree to profess allegiance to various militant Islamist groups.

Federal investigators have said he was likely self-radicalised and there was no evidence that he received any instructions or aid from outside groups such as Islamic State. Mateen, 29, was a US citizen, born in New York of Afghan immigrant parents.

"He appears to have been an angry, disturbed, unstable young man who became radicalised," President Barack Obama told reporters.

Mateen, who was a security guard, was systematic during his rampage, working his way through the packed club shooting people who were already down. He apparently wanted to ensure they were dead, said Angel Colon, a wounded survivor.

"I look over and he shoots the girl next to me and I was just there laying down and thinking: 'I'm next, I'm dead,'" he said.

Mateen shot him twice more, one bullet apparently aimed for Colon's head striking his hand, and another hitting his hip, Colon said at Orlando Regional Medical Center, where he is one of 27 survivors being treated.

FoxNews.com, citing an FBI source, said prosecutors were seeking to charge Mateen's wife as an accessory to 49 counts of murder and 53 counts of attempted murder and failure to notify law enforcement about the pending attack and lying to federal agents.

NBC News said Salman told federal agents she tried to talk her husband out of carrying out the attack. But she also told the FBI she once drove him to the Pulse nightclub because he wanted to scope it out, the network said.

Salman's mother, Ekbal Zahi Salman, lives in a middle-class neighbourhood of the suburban town of Rodeo, California. A neighbour said Noor Salman only visited her mother once after she married Mateen.

Noor Salman's mother "didn't like him very much. He didn't allow her (Noor) to come here," said neighbour Rajinder Chahal. He said he had spoken to Noor Salman's mother after the Orlando attack. "She was crying, weeping."

OBAMA SLAMS TRUMP

Obama denounced Donald Trump for his proposed ban on Muslims entering the United States, joining fellow Democrat Hillary Clinton in portraying the Republican presidential candidate as unfit for the White House.

Since the shooting, Trump criticised Obama for not using the term "radical Islamic terrorism" to describe violent Islamist militants.

"What exactly would using this label accomplish, what exactly would it change?" Obama replied. "Someone seriously thinks we don't know who we're fighting? ... There's no magic to the phrase 'radical Islam.' It's a political talking point. It is not a strategy."

Mateen made 911 calls from the club in which he pledged loyalty to the leader of Islamic State, Abu Bakr al-Baghdadi, whose organisation controls parts of Iraq and Syria.

He also claimed solidarity with the ethnic Chechen brothers who carried out the 2013 Boston Marathon bombing and with a Palestinian-American who became a suicide bomber in Syria for Al Qaeda offshoot the Nusra Front, authorities said.

"We could hear him talking to 911 saying that the reason why he's doing this is because he wants America to stop bombing his country. From that conversation from 911 he pledges allegiance to ISIS," said Patience Carter, 20, who was trapped in a bathroom stall at the nightclub as Mateen prowled outside.

Carter, from Philadelphia, read a poem to the media that she said she wrote to help her heal.

"Looking at the blood and debris covered on everyone's faces. Looking at the gunman's feet under the stall as he paces. The guilt of feeling lucky to be alive is heavy," the poem read.

US officials were investigating media reports that Mateen may have been gay but not openly so, and questioning whether that could have driven his attack, according to two people who have been briefed on the investigation and requested anonymity to discuss it.

A former wife of Mateen, Sitora Yusufiy, said her ex-husband had facets of his life that he did not share with his family, such as drinking and going to night clubs.

"He did have a different side to him that he could not open up to his father about," Yusufiy told CNN. She has previously said he was mentally unstable and beat her and that she fled their home after four months of marriage.

The owner of Pulse, speaking through a representative, denied reports that Mateen had been a regular patron.

"Untrue and totally ridiculous," Sara Brady, a spokeswoman for club owner Barbara Poma, said in an email when asked about the claim.

Mateen's father, Seddique Mateen, indicated soon after the attack that his son had harboured strong anti-gay feelings and on Tuesday he told reporters his son had never mentioned being homosexual.

"I don't believe he was a whatever you call it," he said. - Reuters