The wife of the gunman who killed 49 people at a gay nightclub in Orlando, Florida, last year has been arrested in San Francisco, US officials say.
Noor Salman is suspected of aiding and abetting her husband Omar Mateen, who was killed in a shootout with police, as well as obstructing justice.
It is expected she will be sent to Florida, where charges were filed.
Fifty-three people were also wounded by Mateen, who vowed allegiance to so-called Islamic State (IS).
Police interviewed Ms Salman last year, believing she may have been aware of her husband's plans.
She told the FBI her husband had become radicalised in the year leading up to the attack.
He made his pledge of allegiance during the actual assault, which lasted three hours.
Ms Salman, who moved to the San Francisco area after the attack on 12 June, is expected to appear in a Californian court on Tuesday.
Confirming the arrest, Attorney General Loretta Lynch told the MSNBC network: "This is a matter that we continue to take very seriously.
"We said from the beginning we were going to look at every aspect of this case, 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, is there any other accountability that needs to be had here in this case."
Mateen, 29, was from Fort Pierce, a town about two hours' drive south of Orlando.
His father, Seddique Mateen, said his son had become "very angry" after seeing two men kissing in Miami.
Mateen had been interviewed by FBI officers twice in 2013 following inflammatory comments to colleagues asserting ties to IS but no hard evidence was found.
He appears to have been a regular at the club and there are claims he contacted men on gay dating apps, raising the possibility that he was conflicted about being gay and had lashed out.