Mexico City, March 7 Four US citizens who had travelled to the Mexican border city of Matamoros for medical procedures, were assaulted and kidnapped in a case of mistaken identity, an informed official said.
The official told CNN on Monday that the incident occurred on March 3, adding that investigators believe a Mexican cartel likely mistook the victims for Haitian drug smugglers.
The investigators have not identified any concerning criminal history on the part of the Americans, the official added.
In a statement, Mexican President Andres Manuel Lopez Obrador said: "The information we have is that they crossed the border to buy medicines in Mexico, there was a confrontation between groups and they were detained...
"The whole government is working on it."
Meanwhile, the FBI said that the missing Americans, who are yet to be identified, drove into Matamoros on March 3 in a white minivan with North Carolina plates.
There, they were fired upon by unidentified gunmen and were "placed in a vehicle and taken from the scene by armed men", CNN quoted the agency as saying.
A Mexican citizen was killed in the encounter, US Ambassador to Mexico Ken Salazar said.
"We have no higher priority than the safety of our citizens. This is the most fundamental role of the US government. Officials from various US law enforcement agencies are working with Mexican authorities at all levels of government to achieve the safe return of our compatriots," he said on Monday.
The FBI is seeking the public's help finding them and identifying those responsible and also announced a reward of $50,000 for their return and the arrest of those involved.
Matamoros, a city of more than 500,000 people, is located across the Rio Grande from Brownsville, Texas, CNN reported.
The US State Department has issued a "Level 4: Do Not Travel" advisory for US citizens thinking of going to Tamaulipas, citing organised crime "including gun battles, murder, armed robbery, carjacking, kidnapping, forced disappearances, extortion, and sexual assault is common along the northern border".
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, it added.
The US government estimates that hundreds of thousands of Americans cross the border into Mexico each year to receive healthcare services, including prescription drugs.
The majority of Americans cite cheaper costs as the most common reason to get treatment abroad.
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