Hundreds of naked photos and explicit videos of Hollywood stars were not hacked by breaches in the security of Apple's computer systems, the firm said tonight.
The US tech giant said none of the cases it had so far investigated were a result of its iCloud or Find my iPhone systems being hacked but as a result of a "very targeted attack" on the security procedures of celebrity accounts.
The leaking of the private photos of famous female stars sparked initial suggestions that Apple's iCloud service had been compromised to access the images and leak them across the internet.
The photos have led to a backlash against the unknown hacker who is accused of violating the privacy of the women involved. A police investigation is under way to identify the culprit.
Actress Jennifer Lawrence, model Cara Delevingne and former Downton Abbey actress Jessica Brown Findlay are among the stars whose compromising pictures are said to have been posted online.
Spider-Man actress Kirsten Dunst, who is also reportedly a victim of the leak, tweeted: "Thank you iCloud."
But in a statement Apple said tonight: "We wanted to provide an update to our investigation into the theft of photos of certain celebrities.
"When we learned of the theft, we were outraged and immediately mobilized Apple's engineers to discover the source. Our customers' privacy and security are of utmost importance to us.
"After more than 40 hours of investigation, we have discovered that certain celebrity accounts were compromised by a very targeted attack on user names, passwords and security questions, a practice that has become all too common on the Internet.
"None of the cases we have investigated has resulted from any breach in any of Apple's systems including iCloud or Find my iPhone.
"We are continuing to work with law enforcement to help identify the criminals involved."
The statement continued: "To protect against this type of attack, we advise all users to always use a strong password and enable two-step verification. Both of these are addressed on our website at http://support.apple.com/kb/ht4232."
American actress Lena Dunham said the person responsible for leaking the pictures was "a sex offender" and anyone looking at them was "violating these women again and again".
FBI spokeswoman Laura Eimiller said it was "aware of the allegations concerning computer intrusions and the unlawful release of material involving high-profile individuals, and is addressing the matter", adding that it would be inappropriate to make further comment.
A spokeswoman for Hunger Games actress Jennifer Lawrence, who is one of the biggest stars to be caught up in the scandal, said she had asked US authorities to prosecute whoever is posting the photos, which she said were "a flagrant violation of privacy".
Other stars said to have been affected include Avril Lavigne, Cat Deeley and Rihanna, with actress Mary Elizabeth Winstead, whose movies include A Good Day To Die Hard, already acknowledging pictures in which she is featured are genuine.
She wrote online: "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."
Brown Findlay, best known for her portrayal of Lady Sybil in Downton, is said to feature in a video in which she is naked and appears alongside an unnamed man.
A piece of computer code that repeatedly guesses passwords has been found online. The script was posted to software site GitHub, but a message has since appeared saying that Apple had fixed the bug.
According to the post, the script uses the top 500 most common passwords approved by Apple to try to gain access to user accounts. If successful, it would give the hacker full access to the iCloud account, including photographs.
iCloud is Apple's own cloud service, a wireless storage facility that can be used to access files remotely.