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 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 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 radicalized," President Barack Obama told reporters after a meeting of the White House National Security Council.
FoxNews.com, citing an FBI source, said prosecutors were seeking to charge Salman 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.
Orlando massacre may have been a 'self-hate crime,' US official says
US investigators are digging into what motivated a Florida man to slaughter 49 people at a gay nightclub, including evidence he had become absorbed in militant Islamist ideas and reports he might have struggled with his own sexual identity.
The 29-year-old also called police during his rampage to voice allegiance to various militant Islamist groups.
Federal investigators have said Mateen was likely self-radicalised and there is no evidence that he received any instruction or aid from outside groups such as Islamic State.
"He appears to have been an angry, disturbed, unstable young man who became radicalised," President Barack Obama told reporters after a meeting of the National Security Council.
One official familiar with the investigation, who spoke on the condition of anonymity, said investigators were only beginning to delve into the contents of Mateen's mobile phone and other electronic devices.
The source said they believed Mateen browsed militant Islamic material on the internet for at least two years before the Orlando shootings.
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Orlando gunman was laughing during massacre: survivor
The attack on the Pulse nightclub was the deadliest mass shooting in modern US history, and the worst attack on the lesbian, gay, bisexual and transgender community.
Soon after the attack, Mateen's father indicated that his son had strong anti-gay feelings. He recounted an incident when his son became angry when he saw two men kissing in downtown Miami while out with his wife and young son.
Two US officials who have been briefed regularly on the investigation said if it emerged that Mateen led a secret double life or had gay impulses that conflicted with his religious beliefs, it might have been a factor in his motive.
"It's far too early to be definitive, and some leads inevitably don't pan out, but we have to consider at least the possibility that he might have sought martyrdom partly to gain absolution for what he believed were his grave sins," one said.
A performer at Orlando's Parliament House, another gay club, said he had seen Mateen at Pulse occasionally before his rampage, often accompanied by a male friend. He had not seen Mateen in about two years, he said.
"He was fine most of the time but other times, if he was drinking, he'd go all spastic and we'd have to take him out to his car and make him leave," said the performer, Ty Smith, who uses the stage name Aries.
Nightclub owner denies Mateen was a regular patron
A spokeswoman for Pulse's owner, Barbara Poma, denied that Mateen had ever been a patron.
"Untrue and totally ridiculous," spokeswoman Sara Brady said in an e-mail when asked about the claim.
Mateen's father, Seddique Mateen, told reporters gathered outside his home Tuesday morning that his son had never mentioned being homosexual.
"I don't believe he was a whatever you call it," the elder Mateen said.
'I'm dead'
Angel Colon, who was in Pulse with friends when Mateen attacked, described hearing gunfire and falling to the floor, shot in the left leg.
"I couldn't walk at all," Colon told a reporters at Orlando Regional Medical Center, where he is one of 27 survivors being treated. "All I could do was lay down. People were running over me."
Colon said he had a hopeful moment when Mateen went into a bathroom, where he later took hostages, but the gunman then emerged, systematically making his way through the club shooting people who were already down, apparently to ensure they were dead.
"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 striking his hand, and another hitting his hip, Colon recalled.
"I was just prepared to stay there laying down so he wouldn't know I was alive," Colon said. When police drove Mateen back into a restroom, an officer dragged Colon to safety, he said.
Angel Colon, front second from left, a victim of the Pulse nightclub shooting, speaks about his experience while surrounded by family members Source: AAP
