State attorneys general and federal investigators gathered Monday with public policy and antitrust experts to explore the legal grounds on which they could build an antitrust case against social media giant Facebook Inc., according to people familiar with the matter.

New York Attorney General Letitia James, who has emerged as a lead figure in investigations of Facebook by more than 40 attorneys general, organized the event at her office. A spokesman for Ms. James declined to discuss it with The Wall Street Journal.

Representatives...