MEXICO CITY (AP) — Four U.S. citizens have been kidnapped after gunmen opened fire on their vehicle in the northern Mexico border city of Matamoros, the FBI said.

The four had entered Matamoros, across from Brownsville, Texas, on Friday and were travelling in a white minivan with North Carolina license plates.

The FBI San Antonio Division office said in a statement Sunday that the vehicle came under fire shortly after it entered Mexico.

“All four Americans were placed in a vehicle and taken from the scene by armed men,” the office said. The FBI is offering a $50,000 reward for the return of the victims and the arrest of the culprits.

U.S. Ambassador to Mexico Ken Salazar said in a statement Monday that the Americans were kidnapped at gunpoint and that an “innocent” Mexican citizen died in the attack. He did not offer any additional details, but said various U.S. justice agencies were working with their Mexican counterparts to recover the missing U.S. citizens.

Mexico President Andrés Manuel López Obrador said Monday that the Americans had crossed the border to buy medicine and ended up caught in the crossfire between two armed groups.

Matamoros is home to warring factions of the Gulf drug cartel and shootouts there on Friday were so bad that the U.S. Consulate issued an alert about the danger and local authorities warned people to shelter in place. It was not immediately clear how the abductions could have been connected to that violence Friday.

Tamaulipas state police said people had been killed and injured Friday, but did not say how many. The state police said that neither police nor the military were involved in Friday’s shootouts.

“There have been two armed incidents between unidentified civilians,” the state police said Friday on social media. “The exact number of the fallen is being corroborated.”

Victims of violence in Matamoros and other large border cities of Tamaulipas often go uncounted, because the cartels have a history taking bodies of their own with them. Local media often avoid reporting on such incidents out of safety concerns, creating an information vacuum.

A video posted to social media Friday showed armed men loading two bodies into a truck in broad daylight.

Photographs from the scene viewed by the AP show a white minivan with the driver’s side window shot out and all of the doors open sitting on the side of the road after apparently colliding with red SUV. Multiple people were lying in the street beside it surrounded by rifle-toting gunmen.

Their positions appeared to correspond with the video posted online that was taken from another angle, which showed them being dragged across the street and loaded into the bed of a white pickup. One person who was sitting up in the street walks under their own power to the pickup. At least one other appeared to lift his head from the pavement before being dragged to the truck.

The U.S. State Department’s travel warning for Tamaulipas state warns U.S. citizens not to travel there. However, being a border city, U.S. citizens who live in Brownsville or elsewhere in Texas frequently cross to visit family, attend medical appointments or shop. It would also be a crossing point for people traveling deeper into Mexico.

For years, a night out in Matamoros was also part of the “two-nation vacation” for spring breakers flocking to Texas’ South Padre Island. But increased violence there over the past 10 to 15 years frightened away much of that business.

The FBI said the van the victims were driving Friday carried North Carolina license plates, but authorities provided no other details about who they were or where they were from.