NOGALES, Ariz. -- Black Hawk helicopters aren't just something out of a war movie -- the U.S. Border Patrol uses them to track illegal immigrants and smugglers on the U.S.-Mexican border.

• Watch: Border Patrol: View From Above

The helicopters allow agents to find border-crossers in the most remote parts of the desert -- places impossible to reach by car and difficult to reach on foot.

5 Investigates reporter Tammy Leitner recently accompanied some agents on a mission to find border-crossers.

By the time the brief mission was over, the team had detained 16 illegal immigrants.

According to Leitner, the agents spotted people huddled together and hiding in bushes all over one mountain.

"The area where we were flying was so thick with people (that) we were spotting them every few seconds," she said.

Agents on the ground handcuffed groups of people together while the team in the helicopter searched for more.

"You never know what they're going to have, drugs, weapons -- they always outnumber you," one agent said. "All those thoughts are running through your head as you embark on one of these missions."

Once border-crossers are detained, they're escorted in a single-file line out of the canyon and taken into custody.