Noor Zahi Salman told the FBI that she was with him when he bought ammunition and a holster and she once drove him to the gay nightclub, Pulse, because he wanted to scope it out, a report in the NBC News quoted several officials familiar with the case as saying.
30-year-old Salman, Mateen's second wife, told federal agents that she tried to talk her husband out of carrying out the attack, the news website said.
Officials said Salman is cooperating with investigators.
Early Sunday, the 29-year-old Mateen opened fire at a gay nightclub in Orlando, Florida, leaving 49 people dead and 53 wounded, the worst mass shooting in modern US history.
Federal prosecutors have convened a grand jury to investigate Mateen's wife 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 told Fox News.
Salman told investigators that Mateen shared his plans with her to carry out an attack and the shooter may have even called her from the gay club Pulse during the slaughter, a source told Fox News.
US Senator Angus King, who as a member of the Senate Intelligence Committee was briefed on the investigation, told CNN that "it appears she (Salman) had some knowledge of what was going on, visiting some of the other sites."
Earlier, Mateen's former wife Sitora Yusufiy had said that he himself might have been gay but chose to hide his true identity out of anger and shame.
Yusufiy said in media reports that her ex-husband had told her that he frequented nightclubs before their marriage, but that he did not tell her they were gay clubs.
