An alleged Mexican drug cartel member was arraigned in U.S. federal court Wednesday on murder charges from the roadside ambush of two U.S. immigration agents working south of the border.

A spokesman for the U.S. District Court in Washington says Julian Zapata Espinoza entered a not guilty plea and is being held in jail.

The charges included murder and attempted murder for the Feb. 15 mid-day attack along a four-lane highway that killed U.S. Immigration and Customs Enforcement agent Jaime Zapata and wounded colleague Victor Avila as they were driving to Mexico City after a meeting with other U.S. personnel. The shooting in the northern state of San Luis Potosi was a rare attack on American officials in the country fighting violent drug cartels.

Zapata Espinoza is known by the nickname "El Piolin," or Tweety Bird, apparently because of his short stature. He was captured along with five other suspected members of the Zeta cartel during an army raid a week after the shooting and recently was extradited to the United States.

The Mexican army said Zapata Espinoza admitted killing Zapata in what he claimed was a case of mistaken identity, with the Zetas mistaking the ICE agents' Chevrolet Suburban for one used by a rival gang.