AOL has released 20 million search records from 650,000 users, collected between March and May this year.
Google recently fought the US government to resist its request to reveal search records.
The data was released on AOL's research site on Sunday, but quickly removed. The records are still available on sites mirroring the 439Mb download.
Although the data doesn't directly identify individual users, each searcher is assigned an anonymous ID number, making users traceable.
For example, queries by a single user, spotted by blog The Paradigm Shift, included items such as 'how to kill your wife', 'decapatited photos' and 'car crashes3'.
The data will also aid companies looking to spam search engines and get artificially high rankings for search results.
AOL has issued a response apologising for releasing the data.
"This was a screw up, and we’re angry and upset about it. It was an innocent enough attempt to reach out to the academic community with new research tools, but it was obviously not appropriately vetted, and if it had been, it would have been stopped in an instant," said Andrew Weinstein, a spokesperson for AOL.
"Although there was no personally-identifiable data linked to these accounts, we’re absolutely not defending this. It was a mistake, and we apologise. We’ve launched an internal investigation into what happened, and we are taking steps to ensure that this type of thing never happens again."
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