JUNEAU, Alaska—A sightseeing plane crashed Thursday in southeast Alaska, killing all six people on board, the U.S. Coast Guard said. The plane’s emergency alert beacon was activated around 11:20 a.m. when the plane crashed in the area of Misty Fjords National Monument, near Ketchikan, the Coast Guard and Federal Aviation Administration said. A helicopter company reported seeing wreckage on a ridgeline in the search area, and Coast Guard crew members found the wreckage around 2:40 p.m. A Coast Guard helicopter lowered two rescue swimmers to the site, and they reported no survivors, the agency said. The identities of those killed in the crash were not immediately released. The Alaska State Troopers and volunteers from the Ketchikan Volunteer Rescue Squad will coordinate recovery efforts Thursday and Friday. The plane involved Thursday, a de Havilland Beaver, was owned by Southeast Aviation LLC. “Our hearts are shattered at the loss of six people …