The sexual-violence scandal at Baylor University that cost its celebrated football coach his job involved 17 women who reported sexual or domestic assaults involving 19 players, including four alleged gang rapes, since 2011, according to Baylor regents.

Baylor fired coach Art Briles in May for failing to deal with sexual-violence allegations involving his squad, but provided only a vague description of the alleged improprieties at the time. Legions of fans and donors rallied to his cause.

Now, in interviews with The Wall Street Journal, regents who oversee the university are offering for the first time publicly more detailed findings from an outside investigation conducted by Philadelphia law firm Pepper Hamilton LLP. The probe showed some Baylor players allegedly participated in what one regent calls a “horrifying and painful” series of assaults over several years.

In at least one case, Baylor regents said, Mr. Briles knew about an alleged incident and didn’t alert police, the school’s judicial-affairs staff or the Title IX office in charge of coordinating the school’s response to sexual violence.

The disclosures will likely reignite the sexual-assault scandal that for months has swirled around the private Baptist university, which currently boasts the eighth-ranked football team in the country.