HELENA, Mont.—A Montana prison inmate serving a 100-year sentence for killing his wife in 1983 has confessed to strangling, sexually assaulting, and dismembering a 19-year-old Helena woman 15 years earlier, authorities said Friday. Courtney Brooke Atlas, 79, confessed to killing Pamela Ann Dorrington, who disappeared in 1968. Part of her torso was found near the boat dock at Gates of the Mountains Marina on Holter Lake four months later. Atlas, who was 26 at the time, was Dorrington’s landlord. He had always been a suspect, officials said, but there was no forensic evidence tying him to the crime. Atlas confessed on Wednesday, in detail, after being given immunity from prosecution for the crimes, Lewis and Clark County Sheriff Leo Dutton said. Dorrington’s family was consulted before the immunity offer was made. Atlas said he called Dorrington on the morning of Feb. 17, 1968, and said there was a water leak …