Google disclosed the antitrust investigation in a blog post late Friday.
"We recognize that as Google grows, we're going to face more questions about how our business works," Google's deputy general counsel Don Harrison wrote on a blog.
"We look forward to answering [Attorney General Greg Abbott's] questions because we're confident that Google operates in the best interests of our users," he said.
The investigation stems from complaints by three rivals — Foundem, a British price comparison site; New York-based SourceTool, a website run by parent company TradeComet; and Ohio-based myTriggers.
"They claim that Google's algorithms demote their site because they are a direct competitor to our search engine," Harrison said.
But Harrison countered that “the reality is that we don't discriminate against competitors."
He also suggested that Microsoft has been behind various complaints.
Harrison argued that Foundem is backed by an organization funded largely by Microsoft and that both TradeComet and myTriggers were represented by Microsoft antitrust attorneys.
"The important thing to remember is that we built Google to provide the most useful, relevant search results and ads for users. In other words, our focus is on users, not websites," Harrison wrote.
He also said that because Google cannot give every website a high rank, "it’s unsurprising that some less relevant, lower quality websites will be unhappy with their ranking."