The New York State attorney general has expanded his investigation into daily fantasy sport sites, with measures including the issuance of a subpoena to the online media company Yahoo, according to a person familiar with the inquiry.

The attorney general, Eric T. Schneiderman, sought an injunction in state court Tuesday to prohibit the two most prominent daily fantasy sports websites, FanDuel and DraftKings, from operating in the state, saying they were “plainly illegal” and “nothing more than a rebranding of sports betting.”

Yahoo, which is based in Sunnyvale, Calif., and has been gaining ground on FanDuel and DraftKings, has continued to allow New Yorkers to play daily fantasy games, according to an article on its website.

Yahoo, which expanded from season-long fantasy sports into daily games in July, said on its site that it was continuing to take bets from New York users. Yahoo has established itself the clear No. 3 option behind FanDuel and DraftKings in terms of participation, according to SuperLobby, an independent site tracking the industry’s guaranteed-prize games.