It is one of the largest rewards offered by the U.S. government in recent history.
The Obama administration on Wednesday offered a reward of up to $5 million for information leading to the capture of the suspected drug traffickers who shot and killed a U.S. immigration agent and wounded another in Mexico last month.
The State Department said its Narcotics Rewards Program would pay the amount to anyone coming forward with information that results in the arrest of those responsible for the February 15 attack that killed Jaime Zapata and wounded Victor Avila. Both men were U.S. Immigration and Customs Enforcement agents.
"This reward reflects the U.S. government's unwavering commitment to ensuring that all those responsible for the murder of Special Agent Zapata are brought to justice," said Secretary Napolitano. "We encourage anyone with information about this case to contact authorities."
Zapata and Avila, who worked at the U.S. Embassy in Mexico City, were attacked along a highway as they drove back to the capital from Mexico's northern state of San Luis Potosi. Some U.S. officials say it was an intentional ambush and that the gunmen knew who their victims were.
Zapata and Avila identified themselves at U.S. diplomats in the moments before the shooting.
San Luis Potosi is at the center of a power struggle between the Zetas and the Gulf cartel. It is also on the route north used by migrants seeking to reach the United States. Officials say cartels have begun recruiting some migrants to work for the gangs.
The two agents were in a Chevrolet Suburban. Mexico's drug cartels frequently set up roadblocks and ambushes to steal large SUVs and pickups.
Mexican authorities have detained several people in connection with the murder, including suspected Zeta gang member Julian Zapata Espinoza — known by the nickname "El Piolin," or Tweety Bird. Authorities in Mexico said Zapata Espinoza told them gunmen from the Zetas mistook the agents' SUV for that of a rival gang.
Zapata Espinoza's boss, Sergio Antonio Mora, was also arrested though authorities have not said if he was present at the shooting.
Authorities in both countries have said the investigation continues.
Mexico has also announced a reward of up to 10 million pesos, or about $835,000, for information that leads to the arrest of the killers.
The Narcotics Rewards Program was created by Congress in 1986 to help the government identify and bring to justice the major violators of U.S. narcotics laws.
Associated Press writers Alicia A. Caldwell in Washington and Olga Rodríguez in Mexico City contributed to this report.