The investigation into Google has been joined by 48 states as well as the attorneys general for the District of Columbia and Puerto Rico. The states that have not joined the inquiry are Alabama and California, the home to Google and Facebook, Mr. Paxton said at the news conference.

Major antitrust inquiries can take years, whether they result in legal action or not.

The states have been looking at the big tech companies amid rising concerns about their power in markets and their influence in public communication and political debate. The states formed a multistate unit called the Tech Industry Working Group months ago.

They steadily built up support, both getting bipartisan backing and enough states to marshal the legal resources to pursue in-depth investigations and potentially an antitrust suit.

Each investigation has a smaller core group of states, evenly split between Democrats and Republicans. And they are supported by a larger collection of states that agree to join the effort.

“Obviously, we’re just beginning, but this is going to be a thorough investigation,” said Phil Weiser, the Democratic attorney general of Colorado.

Colorado is a member of the core group in both the Facebook and the Google investigations, and Mr. Weiser is an antitrust expert, having served as a federal antitrust official in the Clinton and Obama administrations. “I’m not worried about having the legal and intellectual firepower for these investigations,” he said.