Originally Published: 06 MAR 23 02:03 ET

Updated: 06 MAR 23 12:23 ET

By Polo Sandoval, Rebekah Riess, Rosa Flores, Josh Campbell and Paul P. Murphy, CNN

(CNN) -- Four US citizens who were assaulted and kidnapped in northeastern Mexico on Friday are believed to have been targeted by mistake, a US official with knowledge of the investigation told CNN, adding the Americans were in Mexico for medical procedures, according to receipts found in their vehicle.

Mexican President Andrés Manuel López Obrador offered a similar explanation, saying Monday the victims had crossed into the border city of Matamoros to purchase medicine.

"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, adding, "The whole government is working on it."

The missing Americans drove into Matamoros on Friday and were fired upon by unidentified gunmen and were "placed in a vehicle and taken from the scene by armed men," a release from the FBI in San Antonio said.

The agency did not identify the missing victims, but it 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.

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 and has also geolocated the images.

The photos obtained by CNN show a woman looking and then sitting next to three bodies outside a white minivan. All the doors of the van were open.

The woman then appears to have been loaded onto the bed of a white pickup truck, the photos show. Several bodies could be seen 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.

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 for further information on the victims and has also sought comment from the government of Tamaulipas, the Tamaulipas Secretary of Public Security's office and the Mexican Attorney General's Office.

Matamoros, a border city of more than 500,000 people in the Mexican state of Tamaulipas, just across from Brownsville, Texas, has been wracked by violence and organized crime at least since the Mexican Drug War began in 2006. The city is home to the Gulf Cartel, a once-powerful smuggling operation that has splintered into small, competing gangs, according to the Congressional Research Service.

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.

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.

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.

Anyone with information related to the incident is encouraged to call the FBI San Antonio Division or submit a tip online.