LOS ANGELES  - As US federal investigators worked Tuesday to determine who stole and posted nude photos of several female celebrities online, the images continued to be removed from various sites.

Copyright complaints apparently prompted the removal of the image from sharing site Imgur.com and rendered links on the social networking site Reddit inoperable in what experts call an online version of ``whack-a-mole'' that will never fully scrub the intimate photos of Oscar winner Jennifer Lawrence and other stars from the Internet.

Apple said Tuesday that its engineers have determined that hackers breached individual accounts, but didn't obtain general access to a pair of its services, iCloud and Find my iPhone. While the tech giant released the results after it said it conducted 40 hours of investigation, law enforcement inquiries likely will take days or weeks to complete.

The FBI offered no details Tuesday on its efforts to identify the people responsible for stealing the images and posting them, but said on Monday that the agency was aware of the breach and was ``addressing the matter.'' Previous investigations have involved the use of search warrants and digital forensics to determine how hackers obtained everything from Paris Hilton's contact list to nude photos of actresses Scarlett Johansson and Mila Kunis.

Lawrence, a three-time Oscar nominee who won for her role in ``Silver Linings Playbook,'' contacted authorities after the images began appearing Sunday. By Tuesday, a Reddit thread that had been compiling links to images of nude photos of Lawrence and other celebrities had been disabled ``due to a copyright claim.'' Users reported difficulty finding working links to the images on other sites, although they remained active on sites that specialize in online piracy.

Representatives of Twitter, Reddit and Imgur did not respond to messages Tuesday. Apple said it was cooperating with the FBI and urged users to adopt stronger passwords and enable the two-step authentication feature to prevent.

Naked images purporting to be of other stars also were posted, although the authenticity of many couldn't be confirmed.

Mark Rasch, a former federal prosecutor who specialized in computer crimes, said investigators will focus on not only who's responsible for the theft of the photos, but the tools they used and even the idiosyncrasies of how they program.

``There is a digital trail,'' Rasch said. ``What you hope for (is) the people aren't very good at what they do, that they screw up, that they (upset) other hackers. Or that they leave a trail.''

While investigations may span months _ and different continents _ Rasch said sometimes authorities will catch an early break or get a tip that leads them to suspects. The investigations are difficult, he said, but ``It's equally difficult to get away with it scot free.''

In the past decade, federal prosecutors have successfully prosecuted a Massachusetts teenager who hacked Hilton's phone account and posted her contact list online, as well as a Florida man who stole nude photos of Johansson, Kunis and singer Christina Aguilera. The teenager was sentenced to several months in jail, while a federal judge in 2012 ordered Christopher Chaney imprisoned for 10 years for the hack that targeted Johansson.

The people responsible for stealing the Lawrence photos may also be tracked by private investigators who can operate faster than government agents, said Rasch, whose company Rasch Technology and Cyberlaw has conducted similar investigations but is not working on the current data breach.

Lawrence and other stars who were hacked are now confronting on a very personal level a problem that has dogged the entertainment industry for years _ online piracy.

``Even if you can get it taken down, it's likely to pop up somewhere else,'' said F. Jay Dougherty, a law professor at Loyola Law School Los Angeles who specializes in entertainment and intellectual property issues.

Mickey Osterreicher, a media lawyer and general counsel for the National Press Photographers Association, said that while a successful copyright complaint could scrub the images from a site forever, Lawrence and other celebrities will have to remain vigilant and continue filing takedown notices.

``You have to go to each place,'' he said. ``It's kind of like playing whack-a-mole.''