Four military police units from Texas are among as many as 1,500 active-duty troops bound for the nation’s southern border to “bolster security,” military leaders said Friday.

After the Department of Defense announced Wednesday that 1,500 troops would be initially assigned to the border mission, what bases and units they were coming from and what other forces could be included remained questions.

The Defense Department has begun to repatriate illegal immigrants, using military cargo aircraft as the Trump administration steps up its crackdown at the southern border. The first such military flight out of El Paso,

President Trump is sending hundreds of troops to the southern border for a wide-ranging mission that poses new challenges and raises questions about the military’s role in handling migration, traditionally a law enforcement responsibility.