Prosecutors have convened a grand jury to investigate Noor Salman, wife of gunman Omar Mateen, sources quoted by Fox News and Reuters say.
She is reported to have told police she tried to talk her husband out of attacking the Pulse nightclub.
The attack in Florida was the worst mass shooting in recent US history.
Fifty-three people were wounded and six remain in a critical condition.
Prosecutors quoted by Fox News said they were seeking to charge Noor Salman 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.
This article continues at [BBC] Orlando gunman's wife, Noor Salman, 'may face charges'