Four American citizens are missing after being assaulted and kidnapped from a vehicle by unidentified armed men in Mexico, the FBI said in a statement.

The Americans came under gunfire from the men shortly after crossing the border on Friday through the city of Matamoros, in the Tamaulipas state, across from Brownsville, Texas, Special Agent Oliver Rich, who is in charge of the FBI’s San Antonio Division, announced Sunday.

The victims crossed into Mexico driving a white minivan with North Carolina license plates, the FBI said. Shortly after, the gunmen fired upon the passengers in the vehicle, and the four Americans were placed in another vehicle and taken from the scene by the men.

Law enforcement has not released the names of those who they said were kidnapped. But Mexican President Andrés Manuel López Obrador said Monday the four were going to buy medicine and that “there was a confrontation between groups, and they were detained.” 

The conditions of the four were not known on Monday morning.

The U.S. Consulate in Matamoros issued an alert on Friday about a shooting in the city.  The message cited media reports that one person had been killed, but the consulate did not independently confirm the death. 

“U.S. government employees have been instructed to avoid the area until further notice,” the alert reads. 

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$50,000 reward being offered in case

The FBI is seeking the public’s assistance in identifying those responsible for the assault and kidnapping and is offering a reward of $50,000 for the return of the victims and the arrest of those involved. 

The FBI, federal partners, and Mexican law enforcement agencies are investigating, officials said.