Six would-be unauthorized immigrants were rescued by U.S. authorities after they became stuck in a tunnel leading from Mexico to San Diego, California, the U.S. Border Patrol said.

Those rescued were not seriously injured. They were treated at the scene and processed for entering the country illegally, Acting Supervisory Border Patrol Agent Michael Jimenez told CNN Wednesday.

At about 11:15 p.m. Tuesday (2:15 a.m. ET Wednesday), border agents on the American side spotted six people entering a manhole on the Mexican side, Jimenez said.

Alerted to a possible illegal crossing, agents reported to the area and walked along the length of a fence where a tunnel that housed an abandoned gas line runs, he said.

A Border Patrol K-9 team detected a hole on the U.S. side, where they saw one person, Jimenez said. Agents had to dig around the person to extricate that person from the hole.

The San Diego Fire Department then arrived and enlarged the hole and removed a metal cap, allowing for the rescue of four men and one woman who were trapped inside, Jimenez said.

The tunnel was approximately 100 yards long and was 2-and-a-half feet high and the same width, he said.

Agents used a small remote-controlled camera to examine the tunnel, and together with questioning of those rescued, determined that no one else was inside, he said.

At least one of the rescued persons told U.S. officials that he had made arrangements to pay $5,000 to be smuggled into the United States through the tunnel, Jimenez said. The people who were rescued were part of a controlled smuggling incident, he said.

The Border Patrol will guard the exit point of the tunnel until it can be sealed with concrete, he said.