[Theoria]

I think Obama gave them authority, within 100 miles of the border.

The 100-mile border zone were adopted by the U.S. Department of Justice in 1953.

[ssapro]

Under 8 CFR 287 (a) (3), Border Patrol agents are authorized to conduct operations including boarding and searching vessels and vehicles for aliens. This authority extends to a reasonable distance from the border and is defined as 100 air miles under 8 CFR 287 (a) (1). These operations are becoming more common on the northern border.