Gunmen kidnapped four U.S. citizens who crossed into Mexico from Texas last week to buy medicine but were caught in a shootout that killed at least one Mexican citizen, U.S. and Mexican officials said Monday.

The four were in a white minivan with North Carolina license plates. They came under fire on Friday shortly after entering the city of Matamoros from Brownsville, the southernmost tip of Texas near the Gulf coast, the FBI said in a statement Sunday.

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

Mexican President Andrés Manuel López Obrador said Monday the four were going to buy medicine, “there was a confrontation between groups, and they were detained,” without offering details.

A woman driving in Matamoros witnessed what appeared to be the shooting and abduction in broad daylight. She asked not to be identified for fear of reprisal.

The scene illustrates the terror that has prevailed for years in Matamoros, a city dominated by factions of the Gulf cartel who often fight among themselves. Amid the violence, thousands of Mexicans have disappeared just in Tamaulipas state, where Matamoros is located.

The woman said she saw the white minivan get hit by another vehicle near an intersection, then gunfire rang out.

Another SUV rolled up and several armed men hopped out.

“All of a sudden they (the gunmen) were in front of us,” she said. “I entered a state of shock, nobody honked their horn, nobody moved. Everybody must have been thinking the same thing, ‘if we move they will see us, or they might shoot us.’”

She said the gunmen forced a woman, who was able to walk, into the back of a pickup truck. Another person was carried to the truck but could still move his head.

“The other two they dragged across the pavement, we don’t know if they were alive or dead,” she said.

Mexican authorities arrived minutes later.

A video posted to social media Friday shows men with assault rifles and tan body armor loading the four people into the bed of a white pickup. One is alive and sitting up, but the others seemed either dead or wounded. At least one person appeared to lift his head from the pavement before being dragged to the truck.

Shootouts in Matamoros were so bad on Friday that the U.S. Consulate issued an alert about the danger. Local authorities warned people to shelter in place. It was not immediately clear how the abductions may have been connected to that violence.

U.S. Ambassador to Mexico Ken Salazar said in a statement Monday the Americans were kidnapped at gunpoint and an “innocent” Mexican citizen died in the attack. He said various U.S. justice agencies were working with their Mexican counterparts to recover the missing persons.

Authorities have provided no other details about who the victims were or where they were from.

President Joe Biden had been informed of the situation, White House press secretary Karine Jean-Pierre said Monday. She declined to answer other questions, citing privacy concerns.

Tamaulipas’ chief prosecutor, Irving Barrios, told reporters that a Mexican woman died in Friday’s shootings. He gave no details about her death and did not specify whether she was killed in the same gunfight where the kidnapping took place.

Tamaulipas state police said on social media there were “two armed incidents between unidentified civilians” on Friday.

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 episodes out of safety concerns, creating an information vacuum.

Photographs from the scene viewed by The Associated Press show a white minivan with the driver’s side window shot out and all of the doors open, sitting on the side of a street after apparently colliding with a red SUV.

The State Department warns U.S. citizens not to travel to Tamaulipas. 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’s also a crossing point for people traveling deeper into Mexico.

As the headquarters of the powerful Gulf cartel, Matamoros was once relatively calm. For years, a night out in the city was part of the “two-nation vacation” for spring breakers flocking to Texas’ South Padre Island.

But increased cartel violence over the past 10 to 15 years frightened away much of that business. Sometimes U.S. citizens are swept up in the fighting.

Three U.S. siblings disappeared near Matamoros in October 2014 while visiting their father and were later found shot to death and burned. Their parents said they had been abducted by men dressed in police gear identifying themselves as “Hercules,” a tactical security unit in the city.