POMONA — Police have increased patrols and other efforts in Pomona in response to four fatal shootings in six days, authorities announced Wednesday. Four men between the ages of 18 and 24 have died and another was wounded in the four separate shootings, which took place March 7, Sunday, Monday and Tuesday, Pomona police officials said. “The department’s homicide detectives and Gang Suppression Unit are diligently investigating the incidents and will continue to do so until all persons responsible are brought to justice,” Pomona police Capt. Michael Olivieri said in a written statement. “The department has begun to deploy additional officers on overtime to supplement the investigative and field crime suppression units,” the captain added. Preliminary investigation indicated the shootings were not directly related, Olivieri said, however, “none of the incidents are considered a random act of violence.” No arrests were made, and Pomona police released no suspect descriptions in any of the slayings. The most recent killing took place about 8:22 p.m. Tuesday at an apartment complex in the 1100 block of Murchison Street, according to Pomona police Sgt. Hector Rodriguez. Officers responded to reports of a shooting and discovered 18-year-old Michael Castillo of Pomona lying on the ground, mortally wounded. He was pronounced dead a short time later. A 20-year-old Pomona man, Christopher Cotinola, died Sunday after he was found shot inside an apartment in the 700 block of North Gordon Avenue, Pomona police Sgt. Steve Congalton said. Officers were checking on a report of “shots fired” just after 9:30 p.m. when they found Cotinola suffering from gunshot wounds to his “mid-torso,” Congalton said. He was rushed to a hospital and treated along the way, but died from his injuries. About 24 hours earlier, another man — 19-year-old Joseph Cook — was shot and killed in the driveway of a home in the 2200 block of Carlton Avenue, Pomona police officials said. His city of residence was not available. Cook was pronounced dead at the scene of the 9:32 p.m. shooting, Congalton said, while a second shooting victim, a 19-year-old man, was flown by helicopter to a hospital for treatment of injuries determined not to be life-threatening. The recent spate of killings began March 7, with the fatal drive-by shooting of 24-year-old Kelly Buruan of Los Angeles as he walked with a child and two strollers about 2:20 p.m. in the 900 block of West 9th Street, Pomona police and Los Angeles County Department of Coroner’s officials said. Anyone with information on any of the shootings was asked to contact Pomona police detectives Bebon or Lange at 909-920-2095.