Litigation over a run on stock in GameStop and other companies—which touched off an abrupt trading halt 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, which in recent weeks was hit with at least two class action lawsuits in Pennsylvania, 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.