SAN DIEGO — Authorities have found what they are calling the "longest" smuggling tunnel ever discovered on the U.S.-Mexico border.

In a statement on January 29, the U.S. Customs and Border Protection agency announced that it had uncovered the cross-border tunnel along the U.S. border with Mexico.

The underground tunnel has a total length of 4,309 feet, making it the longest smuggling on the southwestern border found to date.

Officials said that the tunnel was concealed by a small industrial building in Tijuana, Mexico, and stretched over three-quarters of a mile to San Diego, California.

Images shared by the U.S. border officials show that the tunnel is narrow, only two feet wide and roughly five and a half feet high. Officials said they discovered hundreds of sandbags at the end of the tunnel blocking what is suspected to be an old exit to the city of San Diego.

U.S. authorities also uncovered an incomplete offshoot from the main cross-border tunnel leading into the U.S.

The tunnel housed an extensive transport system including an elevator at the entrance and a rail system. The cross-border tunnel was also found to contain an air ventilation system, a drainage system and high voltage electrical cables.

U.S. officials have yet to make any arrests regarding the discovery of the tunnel.

Previously, authorities had found a 2,966-foot-long tunnel in San Diego in 2014.