SAN DIEGO -- A 30-year old Los Angeles woman driving a pickup truck from Mexico was arrested late Friday night at the San Ysidro port of entry after U. S. Customs and Border Protection officers discovered three undocumented immigrants, two of them children, hidden in a compartment under the bed of the truck, CBP officials announced Monday.

The three women, including a 22-year old and her 11-year old sister, as well as a 14-year old, appeared to be unharmed and were detained as material witnesses, according to a CBP news release.

A 10-year old female U.S. citizen who was a passenger in the truck was released to a family relative, CBP officials said.

According to the news release, the green 1997 GMC truck entered the port just before midnight on Friday and was referred to the secondary inspection lot for a more comprehensive examination after officers became suspicious of the driver and the vehicle.

During the secondary inspection, officers discovered a non-factory compartment under the truck bed and noted what appeared to be clothing and strands of a woman’s hair coming from cracks in the compartment.

Once the truck bed liner was removed, the box-shaped sheet metal compartment was exposed. Officers cut out the center of the truck bed and found the three females lying on their backs.

The driver, a U.S. citizen, was transported to the Metropolitan Correctional Center to await arraignment on the alleged smuggling attempt, government officials said Monday.