The US citizens were crossing the border into Matamoros in Tamaulipas state in a white minivan on 3 March when unidentified gunmen shot at them.

"All four Americans were placed in a vehicle and taken from the scene by armed men," the FBI said.

US and Mexican law enforcement agencies are investigating the incident.

Tamaulipas is one of six states in Mexico that the US State Department advises travellers not to visit due to "crime and kidnapping".

The FBI is seeking help from the public and offering a $50,000 (£41,620) reward for information leading to the return of the victims and the arrests of those involved.

The Americans were driving in a van with North Carolina license plates, according to the FBI, which did not identify the US citizens.

Matamoros is located directly across the border from Brownsville, Texas. 

According to the State Department, "Organized crime activity - including gun battles, murder, armed robbery, carjacking, kidnapping, forced disappearances, extortion, and sexual assault - is common along the northern border", including in the state of Tamaulipas.

Criminal groups have targeted buses as well as cars driving through the state, often taking passengers and asking for ransom payments in an area where local law enforcement has limited ability to respond to crime, the State Department said. 

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