A truck driver was arrested on May 6 after agents discovered six undocumented immigrants hidden in his trailer.

The incident occurred at around 7 a.m. when the driver, 46, arrived at the Highway 86 checkpoint.

While agents were checking his documentation, a Border Patrol canine alerted the team to the truck’s tarp-covered trailer. When agents removed the tarp, they found five men and a woman in the cargo area of the trailer.

The driver was arrested on suspicion of alien smuggling.

All six of the undocumented passengers were returned to Mexico, according to the Border Patrol news release.

The discovery was the sixth case of attempted immigrant smuggling in a tractor-trailer this year in the Imperial Valley.

In April, agents discovered 21 people packed in a utility trailer in the El Centro area. Four similar smuggling attempts were discovered in January, February and March.