The Border Patrol has discovered a secret tunnel that may have been used for smuggling drugs across the U.S.-Mexico border east of San Diego.

Agency spokesman Matt Johnson says an agent heard noise Monday from a 30-foot trailer parked 150 feet north of the border in the town of Tecate.

When he peered inside, he saw a pistol-toting man disappear into a hole. The trailer was half-full of packages believed to contain narcotics.

The Border Patrol says it's unclear if the tunnel was completed. U.S. and Mexican authorities are investigating.

Authorities have discovered dozens of tunnels along the U.S.-Mexico border in recent years, with many clustered around San Diego and Nogales, Arizona.