Federal prosecutors convene grand jury to investigate wife of Orlando massacre gunman, source says

Federal prosecutors have convened a grand jury to investigate the wife of the Orlando nightclub shooter, seeking to charge her as an accessory to 49 counts of murder and 53 counts of attempted murder as well as with failure to notify law enforcement about the pending terrorist attack and lying to federal agents, a federal law enforcement source confirmed Tuesday to FoxNews.com.

FBI agents have interviewed Omar Mateen's wife, Noor Salman, in the days since Sunday's massacre that killed 49 and wounded 53 more.

A federal law enforcement source additionally told Fox News that Salman knew of her husband's deadly plans and did nothing to stop him.

Mateen died in a shootout with police early Sunday morning. While Salman has been widely referred to as Mateen's wife, the only proof of their marriage is her listing as his wife on a 2013 mortgage document and family members who said the two were married and had a 3-year-old son.

Salman told investigators that Mateen shared his plans with her to carry out an attack and Mateen may have even called her from the gay club Pulse during the slaughter, a source told Fox News. Salman also confirmed details of Mateen's gun purchases prior to the assault.

A separate source told Fox News that Salman was cooperating with investigators and had shared information about her husband's violent aspirations. She had taken a polygraph, CBS reported.

Salman is likely to be arrested, a source told Fox News.

Mateen allegedly scouted an alternate location for the shooting -- Downtown Disney -- in April and again in early June, CBS reported.

Fox News also learned on Tuesday that investigators seized two phones that belonged to Mateen, one of which was a Samsung mobile device.

A law enforcement source told Fox News the FBI was able to access both devices, and that investigators are currently going through the data on the phones.

The second device was described to Fox News as an older model, although the exact make could not be specified.

Fox News reported previously that investigators had been scrubbing computers in Mateen's possession for evidence of any communications with individuals known to FBI, photos and video of areas he may have been scouting for attack, as well as traces of terrorist propaganda.

Meanwhile, the FBI is investigating reports that Mateen visited Pulse several times before the attack, a U.S. official told The Associated Press on Tuesday. A law enforcement source told Fox News that Salman drove Mateen to Pulse on at least one prior occasion.

One man also said he recognized Mateen from an app used to arrange dates and hookups for gay men, adding a new layer of complexity to the investigation of the worst mass shooting in modern American history. The U.S. official said the FBI would look into that claim as well.

The Orlando Sentinel cited four Pulse regulars who said they had seen the 29-year-old Mateen there before.

"Sometimes he would go over in the corner and sit and drink by himself, and other times he would get so drunk he was loud and belligerent," said Ty Smith, who claimed to have seen the gunman at Pulse at least a dozen times.

"We didn't really talk to him a lot, but I remember him saying things about his dad at times," Smith added. "He told us he had a wife and child."

Smith's husband, Chris Callen, told the Canadian Press that Mateen had been to Pulse regularly for "at least three years."

Jim Van Horn, 71, told the Associated Press he was a frequent patron at Pulse and said another "regular" there was Mateen.

"He was trying to pick up people. Men," Van Horn said late Monday outside the Parliament House, another gay club.

Van Horn said he met Mateen once. He said the younger man was telling him about his ex-wife.

"My friends came out from the back and said, 'Let's go take pictures on the patio,'" Van Horn said. "So I left. And then they told me they didn't want me talking to him, because they thought he was a strange person."

At around 1 a.m. Sunday, Kevin West was dropping a friend off at Pulse when he saw Mateen crossing the street. West told the Los Angeles Times Mateen had messaged him on and off for the past year using the gay chat and dating app Jack'd.

"He walked directly past me. I said, ‘Hey,’ and he turned and said, ‘Hey,'" West said. “I could tell by the eyes."

About an hour later, wielding an AR-15 semi-automatic rifle and a handgun, Mateen began a three-hour shooting rampage and hostage siege that ended with a SWAT team killing him. During the attack, he called 911 to profess allegiance to the Islamic State (ISIS) group.

In contrast, the Wall Street Journal reported that Mateen had expressed anti-gay views that horrified his classmates at Indian River State College, where he received an associate's degree in 2006. A person who worked at the school during that time said some students would try to "educate" Mateen on his views.

The Journal also reported that Mateen never discussed religion during class.

At Pulse, Smith and Callen told the Canadian Press they had decided to keep their distance from Mateen after an incident during which he pulled out a knife after taking offense to a joke told by one of the couple's friends.

"He said if [the friend] ever messed with him again," Callen said, "you know how it'll turn out."

Fox News' Rick Leventhal  and Matt Dean, FoxNews.com's Malia Zimmerman, and The Associated Press contributed to this report.