Gregory Elarms, 58, has been charged with first-degree murder in the killing of activist David Lewis, who was gunned down near his car in the parking lot of the Hillsdale Shopping Center on June 9.
The district attorney's office has additionally charged Elarms with illegal possession of a firearm and a special circumstance of lying in wait, which could make him eligible for the death penalty.
Elarms was arrested on Dec. 20, two days after he contacted San Mateo police with a tip in the case, which investigators said made him a suspect, according to the district attorney's office.
Investigators believe Elarms followed Lewis to the shopping center from the San Mateo Medical Center, where Lewis was conducting HIV testing and counseling.