Washington: Omar Mateen's wife, Noor Salman, knew of her husband's plans to kill people at an Orlando gay nightclub, a report said on Tuesday.
News agency Reuters quoted a law enforcement source as saying Noor could soon be charged in connection with the deadliest mass shooting in modern US history.
The source added that a federal grand jury had been convened and could charge Noor as early as Wednesday.
In a horrific act, Mateen killed 49 people at the Pulse club early on Sunday.
"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, 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 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 radicalised," 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 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 Noor 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.
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.
Noor'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 neighbor Rajinder Chahal. He said he had spoken to Noor Salman's mother after the Orlando attack. "She was crying, weeping."
(With Reuters inputs)