A deputy police chief was shot dead inside his house in the Mexican border city of Tecate on Tuesday.
Local media said the killing of Jose Soriano, 35, was linked to Monday's discovery in the U.S. of a secret tunnel that may have been used for smuggling drugs across the border. A spokeswoman for the Baja California state attorney general's office, who was not authorized to give her name, would not comment on possible motives.
Soriano was shot several times by an unknown number of gunmen inside his bedroom, where investigators found 45 spent bullets, state prosecutors said in a news release.
On Monday, a U.S. Border Patrol agent found a secret tunnel and an estimated 10,000 pounds (4,535 kilograms) of marijuana inside a nearby trailer in the town of Tecate, California, across the border from the Mexican city of the same name.
The U.S. 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.