The guys opened fire on the Americans shortly after they crossed the border on Friday through Matamoros, in the state of Tamaulipas, across from Brownsville, Texas, according to the FBI's announcement on Sunday.
Using a white minivan with North Carolina license plates, the victims entered Mexico, according to the FBI. The four Americans were put in another vehicle and driven away from the site by the guys before the gunman opened fire on the passengers in the car.
The identities of individuals who were allegedly kidnapped have not been made public by law enforcement. The reason the Americans entered the nation was not immediately clear.
The FBI is asking for help from the public in locating those responsible for the assault and kidnapping, and it is offering a reward of $50,000 for information leading to the capture of those responsible and the release of the victims.
Investigations are being conducted by the FBI, federal partners, and Mexican law enforcement, according to officials. Due of crime and kidnapping in Tamaulipas, the U.S. State Department has issued a "Level 4: Do Not Go" recommendation for American residents there.
