SAN DIEGO -- The Department of Homeland Security said Friday it has set up a toll-free number for information about the recent discovery of the longest tunnel ever found under the U.S.-Mexico border.

Investigators are seeking information from anyone who knows about the construction and use of the sophisticated 2,400-foot tunnel that ran from a warehouse near the Tijuana, Mexico airport to another warehouse in San Diego. Agents are also interested in any leads involving other undiscovered cross-border passageways.

ICE agents believe the tunnel's construction began around May 2004 in Mexico and became operational last November.

The bilingual toll-free tip line started overnight can be reached round-the-clock at (877) 9TUNNEL.

Last weekend, investigators arrested Carlos Cardena Calvillo, a 44-year-old Mexican citizen, who worked at the San Diego warehouse where the tunnel ended, on charges of conspiracy to import marijuana. U.S. Immigration and Customs Enforcement agents are worried that the drug cartel behind the tunnel may be seeking to kill witnesses who were involved with its use or construction.