Fourteen Mexican soldiers were detained by U.S. officials for several hours early on Saturday after they entered U.S. territory at El Paso, Texas, across the border from Ciudad Juárez, Chihuahua.

The soldiers, who crossed the bridge between the cities in two military vehicles, said they didn’t realize they had entered the United States.

The newspaper El Heraldo Chihuahua reported they were from southern Mexico and had only recently arrived at the northern border.

Border agents secured weapons and equipment for “safety and processing,” said U.S. Customs and Border Patrol (CBP), adding that Mexican military leadership was contacted and arrived shortly after.

Thirteen of the soldiers were processed without incident, but one was given a civil penalty after CBP officers discovered a small quantity of marijuana in his possession.

The soldiers appeared to have been handcuffed according to Reuters. Their vehicles and weapons were returned to them when they were released.

The border crossing was closed for about two hours due to the incident, a witness said.

One witness, who asked not to be named, described the tense moments when border officials stopped the soldiers. “The CBP yelled at the soldiers to put their hands up and drop their weapons immediately.”

Involuntary border crossings by Mexican military forces have occurred in the past, the newspaper El Universal reported, but usually in areas where the border is not clearly marked, rather than on international bridges.

With reports from Reuters, El Heraldo Chihuahua and El Universal