Mexico City ? Mexico’s most ruthless drug lord likely was killed by a rival gang aided by corrupt police officers, not in a shoot-out with police as has been widely reported, according to a senior U.S. law enforcement official.

The official, who asked not to be identified but is familiar with the case, said Ramon Arellano Felix, the enforcer of the Tijuana drug cartel run by his family, is believed to have been forced to his knees and executed by gunmen for rival drug lord Ismael Zambada Garcia on Feb. 10 in Mazatlan on Mexico’s Pacific coast.

Exact details of the killing probably never will be known, but he said U.S. officials believe this version is correct.

The death of Arellano Felix and the arrest of his brother, Benjamin Arellano Felix, on March 9 have been portrayed by President Vicente Fox and others as major victories for Mexican law enforcement.

The U.S. official stressed that the capture of Benjamin Arellano Felix, who was thought of as the family cartel’s leader, was indeed a significant accomplishment for Mexican authorities.