PHILADELPHIA — An anonymous social media account has been identifying witnesses to violent crimes across Philadelphia with the stated intention of trying to “expose rats,” and the police are trying to find out who is behind it.

The “rats215” account on the photo-sharing website Instagram has posted pictures, police statements and testimony identifying more than 30 witnesses since February.

Lt. John Walker of the Philadelphia Police Department said investigators learned of the account this month when an officer monitoring Twitter spotted photographs of a witness and court records about an attempted shooting last year. That led to the Instagram account, where officials found witness statements from a 19-year-old victim who said he was shot at because he had testified in a homicide case. The Philadelphia Inquirer first reported news of the Instagram account.

It remains unclear how rats215 obtained the victim’s testimony. The case was handled by a secret grand jury — a practice the Pennsylvania Supreme Court authorized last year to protect witnesses in violent crime cases.