About 100 Mexican soldiers and immigration agents raided a freight train in the southern state of Chiapas on Thursday and detained dozens of Central American migrants riding atop the cars.

Such raids had been rare since the last crackdown on migrants in 2014. But under increasing U.S. pressure to reduce the flow of hundreds of thousands of Central Americans through Mexican territory, Mexico’s government has stepped up enforcement.

At least some of the troops wore armbands of Mexico’s newly formed National Guard. The government says it has deployed thousands of Guard agents across the country with supporting immigration enforcement.

In a scene filmed by Associated Press journalists, the train rolled to a stop in a rural area, and then soldiers climbed ladders to the top of freight cars shouting, “This is the army, you’re surrounded!”