The FBI said that four U.S. citizens have been kidnapped after gunmen opened fire on their vehicle in the northern Mexico border city of Matamoros. The four Amricans had entered Matamoros, across from Brownsville, Texas, on Friday and were travelling in a white minivan with North Carolina license plates. U.S. officials also confirmed on Monday that a Mexican citizen was killed in the incident.

Matamoros is home to warring factions of the Gulf drug cartel, and the U.S. Consulate issued an alert about shootouts there on Friday were so bad that local authorities warned people to shelter in place. The alert reminded U.S. citizens that this part of Mexico is a “Level 4: Do Not Travel,” which is the highest-level warning in the U.S. State Department’s travel advisory system.

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 FBI is offering a $50,000 reward for the return of the victims and the arrest of the culprits.” A post on Twitter appears to show the moment they were kidnapped, with one woman, walking on her own, forced into a white pickup truck. Men armed with guns and wearing bulletproof vests are then seen dragging one person after another into the vehicle. The video posted online appears to show some of them may be injured, but their conditions of the four remain unknown.

On Monday, U.S. Ambassador to Mexico Ken Salazar said in a statement that the Americans were kidnapped at gunpoint and that 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 U.S. citizens, but did not provide further details.  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.

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