“No entity in the history of the world has collected as much information on individual consumers as Google,” he said in a news conference. “We should not just accept the word of these corporate giants that they have our best interests at heart. We need to make sure that they are actually following the law, we need to make sure that consumers are protected, and we need to hold them accountable.”

The investigation comes as once overwhelmingly positive public opinion about tech companies has started to shift. As technology encroaches on more industries and profits pile up at the biggest technology companies, critics increasingly say these firms are too powerful and need regulatory oversight.

Mr. Hawley said the state’s preliminary investigation had found that Google may be collecting more information from users than the company was telling consumers and that users didn’t have a “meaningful option” to opt out of Google’s data collection. Google has said it provides consumers with the option to control their privacy settings and does not provide third parties with personally identifiable information like names, email addresses and billing information.

Patrick Lenihan, a Google spokesman, declined to say how the company intended to respond to the subpoena because it had not received it yet. He said, however, that Google had “strong privacy protections in place for our users” and that it continued “to operate in a highly competitive and dynamic environment.”

In 2012, the F.T.C. voted unanimously to close an investigation into Google’s business practices without bringing charges. As part of the settlement to end the investigation, Google agreed to stop using photos or user reviews from third-party sites that asked that their content not be “scraped” by Google’s computers.