A criminal investigation is under way into a break-in that took place last week on a Microsoft Web site for beta testers, Microsoft said Thursday.

The site, BetaPlace.com, is aimed at people who test Microsoft products. Available at BetaPlace are unreleased versions of Windows and other Microsoft products, as well as keys to activate the software, said one beta tester, who asked not to be named.

Log-in credentials for at least one BetaPlace user were leaked and posted on the Internet, allowing nontesters to download unreleased versions of Microsoft software, according to the beta tester.