Murder charges were filed Tuesday against a disgruntled former patient in a Feb. 29 shooting at a California dental office that killed a dentist and injured two others, the San Diego County district attorney announced Tuesday.

El Cajon resident Mohammed Abdulkareem, 29, pleaded not guilty to one count of murder and two counts of premeditated attempted murder. He is being held without bail.

Dr. Benjamin Ariel Harouni, 28, suffered multiple gunshot wounds when Abdulkareem allegedly burst into the Smile Plus Dentistry office in El Cajon at abut 4:15 p.m. on Feb. 29 and fired at least 23 shots with a semiautomatic handgun, authorities said . Harouni died at the scene.