The official said Noor Salman was taken into custody Monday morning in the San Francisco area and is facing charges in Florida including obstruction of justice, aiding and abetting Omar Mateen's attempted provision and provision of material support to ISIS. A Twitter post from the United States attorney's office in Orlando said Salman will make her initial appearance Tuesday morning in Oakland, California.
The official was not authorized to discuss the matter publicly and spoke on condition of anonymity.
Noor Salman moved to the San Francisco area after her husband, Omar Mateen, was killed in a shootout with SWAT team members during the June 12 massacre at the Pulse nightclub in Orlando.
FBI agents repeatedly questioned Salman in the aftermath of the shooting about whether she had advance knowledge of her husband's plans. She told The New York Times in an interview published last fall that she was unaware that Mateen planned to shoot up the nightclub.
"We said from the beginning, we were going to look at every aspect of this, of every aspect of this shooter's life to determine not just why did he take these actions - but who else knew about them? Was anyone else involved?" Attorney General Loretta Lynch said in an MSNBC interview on Monday.
He was the only shooter, and by the time a three-hour standoff between Mateen and law enforcement had ended, 49 patrons were killed and another 53 people required hospitalization.
Mateen pledged allegiance to the Islamic State group in a 911 call to emergency officials during the standoff. He also made a series of Facebook posts and searches before and during the attack.
The Times first reported on the arrest.
"I want to thank the hard working law enforcement officers from the local, state and federal levels for their continued dedication to help find answers to this horrific terrorist attack that killed 49 innocent people," Florida Gov. Rick Scott said in a statement.
"I hope today's news provides some comfort to the families who are mourning their loved ones," he added.
Last month, Salman filed a petition in a California court to change the name of the son she had with Mateen.
The CNN Wire contributed to this report.