The state attorneys general in more than a dozen states are preparing to begin an antitrust investigation of the tech giants, according to two people briefed on the discussions, increasing pressure on the companies.

The bipartisan group of attorneys general has not yet issued civilian subpoenas — known as civil investigative demands — to the companies. But state investigators plan to do so soon, though the precise timing is not yet set, said the two people familiar with the states’ plan.

A smaller group of the state representatives met last month in Washington with antitrust officials at the Justice Department, which has started a broad antitrust review of the technology industry. The Federal Trade Commission has begun an antitrust investigation of Facebook, and it is reviewing the actions of other companies as well.

Lawmakers in Congress are investigating the industry as well. An antitrust hearing last month included executives from Apple, Amazon, Facebook and Google.