The Justice Department is investigating a stock surge in GameStop—along with several others popular on the Reddit forum r/WallStreetBets—for possible market manipulation, the Wall Street Journal reports. Robinhood, the retail-stock app used by many Reddit traders, has received a subpoena in the investigation. Over the course of January, GameStop stock went from $20 a share all the way up to a peak of $438; it now has fallen below $50. The Securities and Exchange Commission is also investigating the matter, according to the Journal.