The Americans are believed to have been targeted by mistake and were not the intended victims, the official said. Investigators believe a Mexican cartel likely mistook them for Haitian drug smugglers, the official said, adding investigators have not identified any concerning criminal history on the part of the Americans.

The Americans had traveled to the border city of Matamoros for medical procedures, the official said, citing receipts found in the vehicle.

Mexican President Andrés Manuel López Obrador offered a similar explanation.

“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 president said. “The whole government is working on it.”

The incident highlights the ongoing violence in some Mexican cities, which have been wracked by organized crime at least since the Mexican Drug War began in 2006, as well as the growing business of what’s known as “medical tourism.”

The missing Americans, who were not identified, drove into Matamoros on Friday in a white minivan with North Carolina plates, the FBI in San Antonio said. There, they were fired upon by unidentified gunmen and were “placed in a vehicle and taken from the scene by armed men,” the FBI said.

An innocent Mexican citizen was killed in the incident, US Ambassador to Mexico Ken Salazar said.

“We have no higher priority than the safety of our citizens,” Salazar said. “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.”

The FBI is seeking the public’s help finding them and identifying those responsible. The agency announced a reward of $50,000 for their return and the arrest of those involved. The FBI said it is cooperating with other federal partners and Mexican law enforcement agencies to investigate the kidnapping.

CNN has reached out to the FBI, the government of Tamaulipas, the Tamaulipas Secretary of Public Security’s office and the Mexican Attorney General’s Office for more information.

Photos show car crashed prior to incident

Two vehicles rest in Matamoros, Mexico, at the scene which a US official said is connected to the missing Americans.

Photos obtained by CNN show the car believed to have been driven by the Americans crashed with another vehicle before they were taken at gunpoint from the scene.

The FBI would not confirm the authenticity of the images, but CNN has confirmed the authenticity of the photos and video with a US official with knowledge of the investigation. CNN has also geolocated the images.

The photos show a woman looking and then sitting next to three unmoving people lying on the ground outside a white minivan. All the doors of the van were open. It is unclear whether the four people in the photos were the US citizens.

The woman then appears to have been loaded onto the bed of a white pickup truck, the photos show. Several unmoving people could be seen lying on the street next to the pickup truck, the photos show.

One photo shows that an ambulance arrived, but it’s unclear if medical attention was being provided.

Matamoros, a city of more than 500,000 people in the Mexican state of Tamaulipas, is located just across the Rio Grande from Brownsville, Texas. The US State Department has issued a “Level 4: Do Not Travel” advisory for US citizens thinking of going to Tamaulipas, citing crime and kidnapping.

“Criminal groups target public and private passenger buses, as well as private automobiles traveling through Tamaulipas, often taking passengers and demanding ransom payments,” the State Department advisory says.

On the same day as the alleged kidnappings, for example, police issued a warning to parents to keep their kids home from school due to two shootings in the city.

Matamoros was also the site of a large tent encampment of migrants – mostly Venezuelans and Haitians – hoping to cross into the US to request asylum.

In addition, Mexico is a popular destination for “medical tourism,” the term for traveling to another country for medical care, generally for lower costs or for an unapproved procedure.