The Orlando nightclub shooter's wife could soon face charges in connection to the deadliest mass shooting in modern US history, a law enforcement source said on Tuesday (14 June). The FBI source told Fox News that prosecutors have convened a grand jury to investigate Omar Mateen's wife, 27-year-old Noor Zahi Salman.

FBI agents have since interviewed Salman since the attack, which killed 49 and injured 53 others on Sunday (12 June). According to Fox News, the unnamed source claims Salman knew of her husband's attack plans but did not do anything to stop him. Salman reportedly drove with Mateen to the LGBT nightclub, at least once previous to the shooting on 12 June, according to the Orlando Sentinel.

Salman also accompanied her husband to the St Lucie Shooting Center a few days prior to the attack, when he bought a 9mm semiautomatic pistol and a .223-caliber assault rifle. Mateen launched a massive attack at Pulse nightclub in the early hours of 12 June and took several hostages before being killed in a police shootout three after the start of the shooting.

A federal law enforcement official, who spoke to the Sentinel, said there is no evidence so far that Mateen had any accomplices. The FBI said on Monday (13 June) that his wife was not cooperating, but by Tuesday (14 June) that theory had changed. CBS also reported she had taken a polygraph test.

According to Fox News, Salman told investigators her husband had shared his plans with her about the attack and may have even called her from the nightclub during the massacre. It was earlier reported that Mateen allegedly scouted a different location for his attack—Downtown Disney—in April and June, 2016.

Several Pulse nightclub regulars, told reporters that Mateen was a regular at the club, while a man claimed he recognised Mateen from a gay dating app. Ty Smith, who claimed to have seen Mateen at Pulse at least a dozen times, said, "We didn't really talk to him a lot, but I remember him saying things about his dad at times. He told us he had a wife and child."

The FBI source told Fox News that prosecutors are seeking to charge Salman as an accessory to 49 counts of murder and 53 counts of attempted murder plus, failure to notify law enforcement about the attack and lying to federal agents.

Mateen made a 911 call during his rampage and pledged allegiance to the Islamic State. However, federal investigators say he was likely self-radicalised and there is no evidence he received instruction or aid from outside terrorist groups, Reuters reported.

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

Investigators have only just begun looking into the contents of Mateen's cellphone and other electronic devices, Reuters reported. Mateen reportedly browsed militant Islamic material on the web at least two years before the Orlando shooting.