NOGALES, Ariz. -- Every day, agents with the U.S. Border Patrol track illegal border-crossers using tiny clues.

• Watch: Border Patrol: Tracking Smugglers (Extended Edition)

A single footprint, a piece of litter and a tiny thread can all help agents find them.

The footprint, for example, can tell agents if the person was an immigrant or smuggler.

According to agent Jose Molina, drug smugglers tend to wear more expensive shoes.

Empty water bottles and cans of tuna fish left at the side of trails also tell agents a person or group has been through.

Smugglers may leave behind other clues, such as a piece of thread snagged on a branch. The thread may have come from the straps of a makeshift backpack used to carry marijuana.

If the backpacks are heavy enough, the straps will leave marks on a border-crosser's body, which helps agents determine if a person, once captured, is an illegal immigrant or a drug smuggler.

Drug smugglers also paint their shoes black so they don't stand out in the desert at night.

Border patrol agents don't always catch their quarry; if a helicopter helps with the search, wind from its blades could blow evidence away.

If an illegal immigrant does not have a criminal record he may simply be deported back to Mexico. And even with drug smugglers, agents have to be able to prove who the drugs belong to -- which can sometimes be tough.