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 killing by a single shooter in U.S. history, a law enforcement source tells Reuters.

Cesar Rodriguez (centre), a friend of Amanda Alvear, who was killed in the Orlando nightclub shooting, is comforted by Lisa Dominguez (right) at a makeshift memorial. (Drew Angerer/Getty Images) The source said 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 U.S. 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 co-operating 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 nightclub early on Sunday, called 911 during his rampage to profess allegiance to various militant Islamist groups.

Federal investigators have said he was likely self-radicalized and there was no evidence that he received any instruction or aid from outside groups such as ISIS.

The 29-year-old Mateen was a U.S. citizen, born in New York of Afghan immigrant parents.

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

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.

Reports Mateen frequented gay clubs

The FBI is also taking a closer look at reports Mateen was a regular at gay nightclubs, including Pulse, and that he had expressed interest in men on some online gay dating sites.

"I believe in my heart that he was a gay man living in a life that he just could not be his authentic self," Rob Domenico, of the Center Orlando, a hub for the LGBT community, told CBC's Susan Ormiston on Tuesday.

Domenico said three people told him they had seen Mateen on the gay hookup app Grindr dating back a year, soliciting them and flirting.

"I recognized him from one of the apps, but I instantly blocked him because he was, like, very creepy in his messages," said Cord Cedeno.

"He's been in that venue several times; that's not his first time going there. I know that for a fact."

'I'm next, I'm dead'

During his rampage, Mateen systematically made his way through the club shooting people who were already down, apparently to ensure they were dead, Angel Colon, a wounded survivor, said Tuesday.

"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,'" Colon said.

Mateen shot him twice more, one bullet apparently aimed for Colon's head instead striking his hand, and another hitting his hip, Colon recalled.

Orlando nightclub shooting victim Angel Colon describes his terrifying ordeal 4:05

Mateen made calls to emergency 911 dispatchers from the club in which he pledged loyalty to ISIS leader Abu Bakr al-Baghdadi.

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

One official familiar with the investigation, who spoke on condition of anonymity, said investigators were only beginning to delve into the contents of Mateen's cellphone and other electronic devices.

The source said investigators believe Mateen browsed militant Islamist material on the internet for two years or more before the Orlando shootings.