Users can search Google News for recent stories from 4,500 news sites. Two months ago, the site started including press releases among the search results.
Releases culled from PR Newswire and Business Wire, the major distributors of releases in the United States, are marked as such on Google.
However, as first reported by British Web site The Register, releases appearing on sites like Yahoo! News, Seafood.com, and Semiconductor Business News were not marked in search results Thursday.
Nathan Tyler, a spokesman for Mountain View, Calif.-based Google, said that the listing of unmarked press releases was "not intentional," and that the company was working to ensure that all press releases are marked.
"Google includes press releases in Google News because we believe they are an additional resource that offers our users a valuable perspective on the genesis of a story," Tyler said.
He noted that press releases only rarely show up on the front news pages of the site, which are built without human intervention by what is essentially a robot news editor.
Google News was launched in September.