On Thursday, authorities seized about 200 pounds of marijuana with a wholesale value of $1 million on the U.S. side of the tunnel, and Mexican authorities seized about two tons of marijuana on their side.

According to 10News, officials from the Department of Homeland Security heard from their sources that the lives of the people who helped build the tunnel, or know about the tunnel, are in danger.

Those fearing for their lives should go to any border checkpoint and ask for an agent from the U.S. Immigration and Customs Enforcement. Protection will be provided to anyone who comes forward, 10News reported.

The building where the tunnel opens is completly blocked off and surrounded by agents, 10News reported.

Investigators searched for fingerprints and DNA evidence inside the large warehouse on the U.S. side, which had a green sign over the door read V&F Distributors LLC. County records showed the building's owner is Helen Park of Long Beach. The phone rang unanswered Thursday at her home.

Forensic scientists are collecting evidence from the tunnel in an effort to find out who might have helped build the tunnel.

John Fernandes, special agent in charge of the U.S. Drug Enforcement Administration in San Diego, said the Arellano-Felix organization and other drug traffickers operating in Tijuana are surely behind the building and operation of the tunnel.

"It's a vast and sophisticated tunnel," Fernandes said. "We know it's used for drug trafficking, obviously. But more than that, the discovery of this tunnel truly illustrates the dangers, the risks of the security and safety concerns of the American public."

The tunnel, believed to be the largest ever found along the southwest border with Mexico, was the third discovered by U.S. authorities in the past two weeks.

"I think it's indicative of the successes that law enforcement is having above ground along the southwest border," Fernandes said, "because when you drive them underground, obviously there are difficulties on moving their product. Whatever type of contraband above ground is severely thwarted."

The tunnel runs about 2,400 feet or the length of eight football fields.

Portions of the tunnel are 50 to 60 feet deep, and are deeper in other parts. The tunnel has a lighting, ventilation and pumping system to keep seeping water out, said Michael Unzueta, a special agent in charge for ICE.

He said no arrests have been made thus far.

"We're chasing down investigative leads throughout San Diego and Southern California," Unzueta said. "At the same time, we're working cooperatively with DEA and ICE (Immigration and Customs Enforcement) and Mexican authorities in Mexico in the Tijuana area. They're also running down leads."

Randy Warrick, assistant chief of the U.S. Border Patrol, said the tunnel discovery was achieved through a collaborative effort of law enforcement on both sides of the border.

"It does speak well for the law enforcement community of the efforts that collaboratively were put forth to deal a crippling blow to some of these smuggling organizations that are operating in Tijuana," Warrick said.

Unzueta said he went into the tunnel, and it became very warm the lower he went.

"It's like being in a mine shaft or a cave," he said. "It's cavernous. Anybody that suffers from claustrophobia, this would be the last place that they want to be. It's really just cored right into the ground through the earth."