In the course of its investigation into the PlayStation Network security breach, Sony discovered a file that makes a clear reference to the “Anonymous” hacking group.

In a letter to the U.S. House of Representatives on Wednesday, Sony said a file named “Anonymous,” containing the words “We Are Legion,” was left behind by the intruders who gained access to the servers of Sony Online Entertainment. “We Are Legion” is the regular tag line used by Anonymous.

Sony Online Entertainment, or SOE, is the company’s game development and distribution arm. The intruders in the SOE breach compromised information on 24.6 million users, as well as 20,000 credit card and bank account numbers.

Sony discovered the SOE breach on Sunday while investigating an earlier attack that compromised information on 77 million accounts from Sony’s PlayStation Network and Qriocity services in April. Anonymous has not been linked to the PlayStation Network breach — the first one detected. However, Sony noted that both breaches took place in the same time frame, which also corresponded with a denial-of-service campaign launched by Anonymous in retaliation for Sony’s lawsuit against PlayStation tinkerer George Hotz.