Photo captured from CNN

Last Friday, 4 U.S. citizens driving in a car with a North Carolina license plate went missing in the border city of Mexico, Matamoros, the FBI said. The FBI has stated that this is likely kidnapping involving guns. The four had entered Matamoros across from Brownsville, Texas on Friday and were traveling in a white minivan, the photo showed.

An innocent Mexican citizen was killed in the incident, US Ambassador to Mexico Ken Salazar said in a statement on Monday. “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 situation and identities of the 4 missing U.S. citizens are unknown at this time. CBS News reports, “One woman, walking on her own, was forced into a white pickup truck. Men armed with long guns and wearing bulletproof vests are then seen dragging one person after another into the vehicle.”

The FBI San Antonio Division office has asked for public help from anyone who has witnessed this incident or knows the whereabouts of the 4 U.S. citizens. The agency announced a reward of $50,000 for their return and the arrest of those involved.

They had traveled to the border city of Matamoros for medical procedures, CNN reports, citing receipts found in the vehicle. 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.