Internet giant AOL has apologised for releasing the search queries of more than 650,000 of its US subscribers.
AOL offers the fourth most popular search engine
The company admitted the release to researchers was "a screw up" and had breached the privacy of its users.
AOL said it was an "innocent attempt to reach out to the academic community with research tools".
Although users' names were not associated with the search terms, fears were raised that the queries may contain personally identifiable data.
It is not clear which researchers were given the data and how they intended to use it.
The AOL search data was posted about 10 days ago but was not widely known outside the research community until blogs began pointing to AOL's research site on Sunday.
AOL removed the file, but not before copies were already circulating on the internet.
The data file had information on 19 million queries and included information on what search terms were used, when the search was conducted and whether the user clicked on any of the results.
"We're angry and upset about it," AOL spokesman Andrew Weinstein said.
He said that AOL should have vetted the data before releasing it.
Ari Schwartz, deputy director of the technology watchdog group Center for Democracy and Technology, praised AOL for responding quickly.
"We're glad to hear that AOL is treating this as a serious incident because it is a serious incident," he told the Associated Press news agency.