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 U.S. history, a law enforcement source said on Tuesday.

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 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 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-radicalized and there was no evidence that he received any instructions or aid from outside groups such as Islamic State. Mateen, 29, 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," 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.