(CNN) -- Two American immigration and customs officers driving across Mexico on Monday were shot and "seriously wounded" by unidentified gunmen, according to U.S officials.

The attack occurred somewhere on the road between Monterrey and Mexico City on Monday afternoon, according to a statement released by U.S. Immigration and Customs Enforcement (ICE).

The identities and precise conditions of the two officers is unknown. However, an ICE official speaking on the condition of anonymity said both agents were "seriously wounded." It is also unknown where the attack occurred on the 567-mile route between the two cities.

The official also said these were the first shootings of ICE agents in Mexico.

"ICE is working with the U.S. State Department, Mexican authorities and other U.S. law enforcement partners to investigate the shooting," the agency statement read "Our thoughts and prayers are with our colleagues."

According to ICE, the agency has about 30 officers assigned to its office in Mexico City. They work a range of issues including drugs, weapons, currency and immigration.

Jeanne Meserve contributed to this report