The Texas attorney general has opened an antitrust investigation into how Google ranks search results, the first United States case to strike at the heart of the company’s main search business.

The issue at hand  referred to as search neutrality  is whether Google manipulates results to thwart competitors and advance its own businesses. Some companies worry that Google has the power to discriminate against them by lowering their listings in search results or charging higher fees for their paid search ads.

“This whole issue of how Google treats these vertical competitors, that’s a big issue now,” said Gary L. Reback, a lawyer at Carr & Ferrell in Palo Alto, Calif., who has advised small companies that have brought antitrust cases against Google.

Google announced the inquiry by the Texas attorney general, Greg Abbott, on Friday after Search Engine Land, an industry blog, reported on it. Lauri Saathoff, a spokeswoman for Mr. Abbott, confirmed the existence of the investigation but declined to give more information because the review was not complete.