Litigation over a run on stock in GameStop and other companies—which touched off an abrupt trading halt on Jan. 28—has expanded beyond Robinhood and its customers, with attorneys now alleging a conspiracy to manipulate the market that involved several other online trading platforms, hedge funds and clearinghouses.

Robinhood halted trades of at least 13 companies last month after Reddit users and others bought up their stock, sending share prices soaring and threatening short sellers with potentially billions of dollars in losses. Most of the lawsuits first filed alleged consumer fraud and breaches of fiduciary duty against Robinhood on behalf of its customers, but the latest class actions brought claims under antitrust law, naming dozens of defendants such as hedge fund Citadel and brokerage firm TD Ameritrade Inc.