Authorities in Mexico say they have arrested a Zetas drug cartel leader suspected of involvement in the killing of a U.S. immigration agent earlier this year.

Officials say police took Jesus Enrique Rejon Aguilar into custody Sunday outside Mexico City without firing a shot. They say Rejon Aguilar, nicknamed “Mamito,” is a founding member of the Zetas and third in command of the organization.

Rejon Aguilar was sought in connection with a February attack in San Luis Potosi state that left U.S. agent Jaime Zapata dead and another colleague traveling with him, Victor Avila Jr., seriously wounded. Mexican officials have said the agents were attacked when they stopped at what seemed to be a military checkpoint, possibly set up by drug traffickers. Mexico’s military has said it had no checkpoints in the area.

The U.S. State Department offered a $5 million reward for information leading to Rejon Aguilar’s arrest.

Attacks on U.S. law enforcement personnel in Mexico are rare, despite increasing U.S. contributions to Mexico’s fight against drug trafficking. The last high-profile attack there was in 1985, when a Drug Enforcement Administration officer was captured, tortured and killed while on an assignment.

Mexico’s military forces are engaged in a brutal struggle against violent drug cartels.

More than 37,000 people have been killed in drug-related violence since President Felipe Calderon took office in late 2006 and began cracking down on the nation’s cartels.