Apple says a "targeted attack" on some user accounts led to the release of nude celebrity photos, but it has found no breach of its cloud storage system.
Private, nude pictures of dozens of female celebrities, including Oscar-winning actress Jennifer Lawrence and top model Kate Upton, were posted on the internet after online storage accounts were hacked.
The images were posted on image-sharing forum 4chan, although the authenticity of many could not be confirmed.
Apple said when it learned of the theft, "we were outraged and immediately mobilised Apple's engineers to discover the source".
"We have discovered that certain celebrity accounts were compromised by a very targeted attack on user names, passwords and security questions," it said.
"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 FBI confirmed it was investigating the hacks.