The Boston-area trader at the center of the past week’s frenzy over GameStop Corp., who also worked until last week as a broker, may face legal jeopardy for potential violations of federal rules governing brokers’ communications with the public, according to securities lawyers.

Keith Gill, who goes by the moniker “Roaring Kitty” on YouTube and other names on websites such as Reddit’s WallStreetBets, has become one face of the GameStop mania because he was an early booster of the stock and forecast its rise. His commentary could also create a legal problem for the Massachusetts Mutual Life Insurance Co., which employed him and had a duty to supervise him.

THIS HEDGE FUND MADE $700M ON GAMESTOP

Mr. Gill worked as a “financial-wellness education director” but was a registered broker, according to BrokerCheck, a website of the Financial Industry Regulatory Authority, or Finra. He didn’t advise specific clients on investments or insurance, but he wasn’t exempt from regulations that tightly regulate brokers’ conduct, securities lawyers said.

Mr. Gill has posted dozens of videos or livestreams over the past six months, most of them related to his view that GameStop shares were undervalued and would rise as others took notice. Some videos examined the company’s past performance and forecast aspects of its future outlook.

Brokerage firms are required to closely supervise their brokers’ communications on social media if those posts have anything to do with their work. Any misleading statements could hurt customers and create liability for the brokerage firm.

Talking or writing under a pseudonym doesn’t absolve a broker of regulatory obligations, said Susan Light, a partner at Katten Muchin Rosenman LLP.

“An average Joe can go on a website and say, ‘I like XYZ stock.’ A broker can’t do that,” said Ms. Light, a former chief counsel for enforcement at Finra.

A state securities regulator has already asked MassMutual about Mr. Gill’s activities on YouTube and Reddit, according to a letter to the firm from the Massachusetts Secretary of the Commonwealth. The Massachusetts office last week told the firm to explain why Mr. Gill’s social-media presence wasn’t reported as an outside business activity, according to the letter, a copy of which was viewed by The Wall Street Journal. It also asked MassMutual whether it monitors its employees’ social-media use. The New York Times earlier reported that inquiry.

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