The couple has been married since 2011. They have a three-year-old son.
Prosecutors quoted by Fox News said they were seeking to charge Salman, who was Mateen’s second wife, as an accessory to 49 counts of murder and 53 counts of attempted murder, as well as with failure to warn authorities about the impending attack.
It was possible that Mateen had called his wife from inside the club while the killings were taking place, Fox quoted a source as saying.
US Senator Angus King, a member of the Senate Intelligence Committee which received a briefing on the investigation, told CNN that “it appears she had some knowledge of what was going on”.
“She definitely is, I guess you would say, a person of interest right now and appears to be co-operating and can provide us with some important information,” he added.
The FBI is investigating reports that Mateen made several visits to the Pulse nightclub and made contact with other men on gay dating apps.
President Barack Obama is scheduled to travel to the scene of the attack in Orlando on Thursday.