Investigators have identified three people as the suspected masterminds of last week’s ambush slaying of 13 law enforcement officers in central Mexico, and a drug gang appears to have been involved, authorities said Tuesday.

The prosecutor’s office in the State of Mexico said 25 others have been detained in the case and are suspected of working for the Familia Michoacana drug gang.

Warrants have been issued for the three purported masterminds, but they apparently remain at large. Authorities offered a reward of about $25,000 each for information leading to their arrest.