Google is facing an investigation by Texas regulators over accusations that it has been modifying search results in an anti-competitive manner.
The authorities have asked for details on three companies as part of their investigation - Foundem, a British price comparison site, SourceTool and myTriggers - all of which have close links with Microsoft.
"We work hard to explain our approach to search and how our ranking works, and we also listen carefully to people's concerns," said Don Harrison, Google's deputy general counsel, in a blog post.
"We're looking forward to working co-operatively with the Texas Attorney General's office, and we strongly believe our business practices reflect our commitment to build great products for the benefit of users everywhere."
Foundem, which is part owned by Microsoft, has already been cited as a complainant in the European Union for a similar investigation, and SouceTool has had a private lawsuit on the matter dismissed in a federal court.
Google said that myTriggers is also represented by Microsoft lawyers, and the company has given evidence that its traffic loss was down to overheating servers.
Google is already facing anti-trust probes by the EU, the US Department of Justice and the Italian government, and there are calls for an investigation in France.
In the US, Consumer Watchdog has called for Google to be broken up to introduce more competition.
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