Weak passwords or a vulnerability in Apple's iCloud platform may have been exploited by a hacker who posted private nude photos of a number of Hollywood female celebrities.
The hacker posted online a laundry list of 101 celebrities from whom he stole nude photos, including Oscar-winner Jennifer Lawrence and pop star Rihanna, and threatened to post them within days.
The leaked photos prompted angry reactions from the stars.
"This is a flagrant violation of privacy. The authorities have been contacted and will prosecute anyone who posts the stolen photos of Jennifer Lawrence," the Hunger Games star's agent said.
By late Sunday, Twitter had begun suspending accounts that linked to the Lawrence photos.
There have been reports that the hacker accessed the files through the iCloud platform, which is used by iPhone users to store settings and, crucially, which backs up photos taken with the phone to "cloud" servers, but this has not been confirmed.
Experts speculate the likeliest explanation was the leak of an email and password combination, either by guesswork or "phishing", when users are fooled by sites into entering their login details, which are then used against them.
Among the celebrities whose pictures were allegedly stolen and posted online were Avril Lavigne, Amber Heard, Gabrielle Union, Hayden Panettiere and Hope Solo.
Media reports said among the other starlets targeted were Hillary Duff, Jenny McCarthy, Kaley Cuoco, Kate Upton, Kate Bosworth, Keke Palmer and Kim Kardashian.
Former Nickelodeon star Victoria Justice said the images claiming to show her nude were fake. But horror movie actress Mary Elizabeth Winstead, whose intimate photos were leaked, acknowledged her frustration.
"Knowing those photos were deleted long ago, I can only imagine the creepy effort that went into this," she tweeted.
Watch: Apple, FBI investigate massive celebrity photo 'hack'
Additional reporting by The Guardian