A U.S. customs official was shot dead and another injured by unknown attackers in Mexico Tuesday, a senior U.S. government official said.

Homeland Security Secretary Janet Napolitano said the official was killed "in the line of duty," and she was saddened by the news.

The two U.S. Immigration and Customs Enforcement agents, who are based in Mexico City, were attacked by "unknown assailants" while driving from the country's capital to the northern city Monterrey, Napolitano said in a statement.

The motive for the shooting is not clear and no one has claimed responsibility for it so far.

The attack occurred on a highway near San Luis Potosi, a city about 320 km north of Mexico City that rarely witnesses drug-related killings.

The secretary said the injured official "remains in stable condition" and the U.S. and Mexican authorities are working closely together to capture the suspect.

Mexico's Foreign Ministry denounced the illegal act and said it would firmly cooperate with the United States in investigations.

Mexico has been fighting drug trafficking and organized crimes which have killed more than 32,000 people since President Felipe Calderon took office in 2006.