MEXICO CITY (AP) — Top officials apologized publicly Thursday for an infamous 2011 massacre in which drug cartel assassins aided by local police seized and killed dozens or perhaps hundreds of people in the northern town of Allende and disposed of their bodies.

Interior Secretary Olga Sánchez Cordero, who presided over the ceremony, acknowledged “grave human rights violations were committed” as well as “omission and complicity by authorities in the face of the crimes committed by organized crime,” according to a statement.

Sánchez Cordero called the massacre in Allende, which lies in Coahuila state about 35 miles (55 kilometers) from the Texas border, “one of the most painful events” in the country’s history.

A group formed to search for missing people in Coahuila and elsewhere in the country said it respects family members who accept the apology but the organization rejects what it considers a pretense, Milenio reported.

The 2011 attacks were believed to have been ordered by Zetas cartel bosses in revenge for a perceived traitor. The exact number of victims who disappeared has never been determined.