The Federal Trade Commission said Thursday it had ordered the ChatGPT maker to provide details on recent partnerships.
The move also covers AI startup Anthropic, and tech giants Microsoft, Amazon and Alphabet.
It comes after deals between a small number of big players raised antitrust concerns.
The orders will now allow regulators to scrutinize the inner workings of the firms involved, to see if they hurt competition.
One legal expert said it was a message to companies that Washington is watching, and ready to act.
The companies now have 45 days to respond.
Microsoft promised to comply, but said all its deals with OpenAI were meant to promote competition.
It's been pouring billions into the ChatGPT maker.
Google said an inquiry would shine a light on firms that were less open than it is.
There was no immediate comment from the other firms affected.
Last June, the FTC published a list of its concerns.
It identified access to data, talent and computational resources as key areas, and vowed to use all its powers to tackle anticompetitive behaviour.
