El Monte, Calif. (AP) -- The lead investigator in the slaying in Mexico of a Southern California school board member has been killed in an ambush, authorities said Saturday.

Mexican officials wouldn't say whether investigator Manuel Acosta's killing was related to the killing of Agustin Roberto "Bobby" Salcedo last month.

Acosta, 42, was ambushed near his office Jan. 15 by gunmen in a pickup truck. He was shot several times in the chest and torso, but survived in critical condition. He succumbed to his wounds Tuesday.

Authorities didn't immediately disclose the attack, saying they hoped to better protect Acosta by letting his assailants assume he was dead. Five others were killed in the attack.

His death was first was reported in Mexico's Milenio newspaper and confirmed to the Los Angeles Times Saturday by Martin Chavez, spokesman for the city of Gomez Palacio.

It's unclear what impact the death of Acosta will have on the case, in which little progress has been reported.

U.S. Rep. Judy Chu, who has pushed for strong response to the Salcedo case, said Salcedo's death was shocking and called Acosta's "doubly shocking."

Chu, whose district includes El Monte, said she would continue to argue for the investigation to be put into the hands of federal authorities in Mexico instead of local ones. Mexican law requires that cases meet certain legal requirements to qualify for federal investigation, she said.

Salcedo, a 33-year-old assistant principal, was killed December 31 while he and his wife were visiting relatives in Gomez Palacio. Salcedo and five other people were abducted from a restaurant and later found shot in the head, their bodies dumped in the outskirts of town.

Salcedo, a Southern California native, is believed to have been the first U.S. elected official killed in the 4-year-old Mexican drug war.

Gomez Palacio is ground zero of the drug war. Authorities suspect a drug cartel was involved in the killings that included Salcedo.

A few days after his death, about 5,000 people gathered for a vigil in Salcedo's honor in El Monte, where he had been a teacher, coach and school administrator.