State police officers stand guard near a crime scene in downtown at a Mexican city on the border with Brownsville, Texas, in Matamoros, Mexico, on Friday.

MEXICO CITY — Four U.S. citizens are missing after they were kidnapped from their vehicle by unidentified armed men in Mexico, the FBI said.

The Americans came under fire shortly after they crossed the border Friday into the city of Matamoros, across from Brownsville, Texas, the FBI said in a statement posted Sunday on the website of the U.S. Embassy in Mexico City.