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 the Reuters news agency on Tuesday 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 groups.

Federal investigators have said that he was probably "self-radicalised" and there was no evidence that he received any instruction or aid from outside groups such as Islamic State of Iraq and the Levant (ISIL, also known as ISIS).

'Angry, disturbed, unstable'

Mateen, 29, was a US citizen, born in New York.

"He appears to have been an angry, disturbed, unstable young man who became radicalised," President Barack Obama told reporters after a meeting of the White House National Security Council.

During his rampage Mateen systematically made his way through the packed club, shooting people who were already down, apparently 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.

Fox News, 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, plus failure to notify law enforcement about the pending attack and lying to federal agents.

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

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A former wife of Mateen, who was a security guard, has said he was mentally unstable and beat her. The ex-spouse, Sitora Yusufiy, said she fled their home after four months of marriage.

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 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.

Trump had criticised Obama for not using the term "radical Islamic terrorism".

"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."

Obama criticised what he called "yapping" and "loose talk" from Republicans over the fight against terrorism.

Jumble of ideologies

Mateen made 911 calls from the club in which he pledged loyalty to the ISIL leader, 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-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.

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One official said investigators believe Mateen browsed "militant Islamist" material on the internet for two years or more before the shootings.

Soon after the attack, Mateen's father indicated that his son had harboured strong anti-gay feelings. He recounted an incident when his son became angry when he saw two men kissing in Miami while out with his wife and son.

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 regularly on the investigation and requested anonymity to discuss it.

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, told reporters his son had never mentioned being homosexual. "I don't believe he was a whatever you call it," he said.