Thirteen Mexican State Police officers have been arrested in connection with the killings of 11 men found in a mass grave in the Northern border city of Juarez.

The arrested police officers have been flown to Mexico City, where they are being questioned by senior officials.

But the attorney general, Rafael Macedo de la Concha, is already conceding that four other officers, including State Police Commander Migual Angel Loya, are fugitives from justice. Mr. Loya failed to show up for work on Monday, and has not been seen since.

Mexican authorities estimate the killings happened about a year ago, and say they are directly connected to the notorious Vicente Carillo Drug Cartel, which operates throughout this region.

They say a bloody internal power struggle is raging, following the arrest of a number of senior drug cartel operatives.

The mass grave was discovered near a house last weekend. A man who rented the house, who was caught trying to flee to the United States, has admitted to murdering some of the men and burying them, on orders from the cartel. He has also implicated some state police officers, and explained that there are more bodies to be found.

Agents of the attorney general's office are searching other locations for more mass graves. Local families have indicated that more than 90 people have been reported missing.

Over the last decade, more than 200 young women have been murdered in and around the city of Juarez, which skirts the border with the United States. Little progress has been achieved in the overall investigation, and last year Mexican President Vicente Fox removed overall jurisdiction for the investigation from local and state police and put federal agents in charge.