Police are investigating the death of a woman found in her Chelsea home early Sunday morning, according to the Suffolk district attorney. Neighbors asked Chelsea firefighters to check on the Washington Avenue resident, whom they had not seen in several days, Jake Wark, spokesman for District Attorney Daniel F. Conley, said by e-mail. The woman’s body was found just before 1 a.m. Wark did not release the identity of the woman, believed to be in her late 80s or early 90s. The state medical examiner will investigate the cause of death, Wark said. He was unable to say whether foul play was suspected or the death appeared to be heat-related, but he suggested area residents check on older relatives and neighbors while high temperatures persist.