NOGALES, Ariz. -- Agents on the U.S.-Mexico border work at a frantic pace -- chasing smugglers, dodging rocks and hunting for illegal immigrants.

• Watch: Border Patrol: The Chase (Extended Edition)

The brush is unforgiving; there are plenty of places for border-crosses to hide.

"Every day this happens," said Agent Daniel Rodriguez. "Every day."

On her first day with the border patrol, 5 Investigates reporter Tammy Leitner saw agents detain dozens of illegal immigrants.

Some told agents they were coming over to find work that they couldn't find in Mexico; a second group said they had been walking for five days, and their guide had abandoned them.

One said she was trying to see her family in New York City. She was five months pregnant.

After detaining the illegal immigrants, agents rushed through the canyons to the border, where someone had seen bundles, possibly containing narcotics, tossed over the wall.

Though the smugglers escaped, agents found the bounty: three 40-pound bundles of marijuana.

Agents say smugglers often strap the bundles onto their backs with makeshift backpacks.

Between calls about border-crossers, agents receive one asking for backup at the border. Apparently, immigrants are throwing rocks at the agents.

These "rockings" happen every day, and some cause severe injuries.

Nevertheless, agents don't catch this group of assailants; by the time they arrive, the immigrants have fled to the Mexican side of the border where the Border Patrol is powerless.

As the sun sets, radios go silent.

It's the calm before the nighttime storm.

E-mail reporter Tammy Leitner at Tammy.Leitner@kpho.com.