A member of the Mexican security forces stands next to a white minivan with North Carolina plates and several bullet holes, at the crime scene where gunmen kidnapped four U.S. citizens who crossed into Mexico from Texas on Friday. Associated Press photo

MEXICO CITY — Four U.S. citizens are missing after they were kidnapped from their vehicle by unidentified armed men in Mexico, the FBI said.

The Americans came under fire shortly after they crossed the border Friday into the city of Matamoros, across from Brownsville, Tex., the FBI said in a statement posted Sunday on the website of the U.S. Embassy in Mexico City. The Americans were traveling in a white minivan with North Carolina plates.

“All four Americans were placed in a vehicle and taken from the scene by armed men,” the FBI said.

Oliver Rich, the FBI special agent responsible for its San Antonio Division, said the bureau is seeking assistance from the public in identifying the kidnappers. The FBI did not name the victims.

The FBI, which is investigating the kidnapping alongside Mexican law enforcement agencies, is offering a $50,000 reward as authorities seek the gunmen’s arrest and the return of the victims.

The bureau’s San Antonio Division declined to give further details about the case or missing Americans on Monday morning. The State Department and Mexican police did not immediately respond to requests for further comment.

Matamoros, home to 580,000 people, is the second largest city in the northeastern state of Tamaulipas, across from Texas’s southern tip. Tamaulipas is one of six Mexican states to which the State Department advises Americans against traveling, citing the risk of 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 says. Heavily armed members of criminal groups often patrol border regions in the state.

On the day of the kidnapping, the U.S. Consulate in Matamoros said it had received police reports of a deadly shooting in the city and ordered U.S. government officials to avoid the area in the vicinity of Calle Primera and Lauro Villara. There was no immediate indication that the incident was connected to the kidnapping.

Tamaulipas is among the most dangerous states in Mexico, largely due to violent conflict between armed groups warring for territory. Some of those groups in recent years have used kidnapping as a revenue stream, sometimes abducting migrants waiting along the border.

But security in Matamoros has improved in recent years, with fewer high-profile attacks like the one that preceded Friday’s kidnapping. Many Americans who live across the border in Brownsville walk or drive to Matamoros for lunch or to see a doctor. In January, Texas Monthly published a list of the city’s best taquerias.

Although Matamoros is firmly in the hands of the Gulf Cartel, fractures within that group have led to waves of violence elsewhere in Tamaulipas state. Analysts have expressed concern that if similar fissures emerge in Matamoros, the city’s relative peace could be disturbed.