(CNN) - A hacker's leak of nude photos of Jennifer Lawrence and other female celebrities is now a case for the FBI.
Nude photographs of Lawrence, who won an Academy Award last year for her role in "Silver Linings Playbook," were splashed on various sites Sunday. They appeared on 4chan, Reddit, Twitter and Tumblr, among others.
Kate Upton's lawyer confirmed photos of the model-actress were among those leaked, calling it "an outrageous violation" of her privacy. "We intend to pursue anyone disseminating or duplicating these illegally obtained images to the fullest extent possible," attorney Lawrence Shire said.
Lawrence's publicist said "authorities have been contacted and will prosecute anyone who posts the stolen photos of Jennifer Lawrence."
Some of the alleged victims in the celebrity hacking case "have engaged with the FBI," a U.S. official told CNN's Pamela Brown Monday.
The Los Angeles office of the FBI successfully investigated another hacker who stole nude photos, scripts and personal information from the e-mail accounts of entertainers including Scarlett Johansson, Mila Kunis and Christina Aguilera in 2011.
"The FBI is aware of the allegations concerning computer intrusions and the unlawful release of material involving high profile individuals, and is addressing the matter," FBI spokeswoman Laura Eimiller said Monday. "Any further comment would be inappropriate at this time."
The previous case, which resulted in a Florida man being sentenced to 10 years in prison, involved hacking through the "forgot my password" function on celebrity e-mail accounts. Cyber-security experts are now pointing the finger at a flaw in Apple's "Find My Phone" app that opened access to celebrities' data stored in iCloud.
Apple confirmed to CNN Monday that it is looking into reports that its popular iCloud online data backup service may have been compromised by the hackers.
"We take user privacy very seriously and are actively investigating this report," Apple spokeswoman Natalie Kerris said.
Actress Kirsten Dunst, one of the hacker's targets, tweeted "Thank you iCloud." She added emoticons that graphically expressed her dissatisfaction with the service.
Celebrity blogger Perez Hilton posted what he said were naked photos of Lawrence and actress Victoria Justice, but he later took them down.
"Upon further reflection and just sitting with my actions, I don't feel comfortable even keeping the censored photos up. I am removing them," Hilton tweeted. "At work we often have to make quick decisions. I made a really bad one today."
The leaked photos sparked an online backlash, with some of the targeted celebrities decrying them as fake while others confirmed they're real.
"To those of you looking at photos I took with my husband years ago in the privacy of our home, hope you feel great about yourselves," actress Mary Elizabeth Winstead wrote.
"Knowing those photos were deleted long ago, I can only imagine the creepy effort that went into this. Feeling for everyone who got hacked."
Some celebrities said the photos were doctored.
Photos claiming to show Ariana Grande are "completely fake," someone close to the singer said. Justice said pictures purporting to be of her are not real, either.
Hacking of celebrity accounts has become more popular in recent years.
Two years ago, a judge sentenced Christopher Chaney of Florida to 10 years in federal prison for hacking the e-mail accounts of 50 entertainment industry figures.
The first real case of a celebrity hacking was in 2005. Hackers logged into Paris Hilton's phone and stole photos of the reality star, said Mikko Hypponen, chief research officer at computer security company F-Secure, in 2011.
At the time, the hackers broke into Hilton's phone by guessing the not-so-secret answer to her security question, which was "tinkerbell" -- the name of her pet Chihuahua.