A medical helicopter transporting a patient crashed off Catalina Island near Los Angeles, killing two people and leaving two others hospitalised, authorities said on Thursday.Rescuers continued searching for a fifth person who was aboard the aircraft and went missing after the Wednesday night crash.The Coast Guard, firefighters and divers with the Los Angeles County Sheriff’s Department were among emergency crews who searched through the night and into the night on Thursday.The cause of the accident was not immediately clear. Coast Guard Captain Stacey Crecy said the helicopter went down shortly after take-off and hit the water a short distance from the island.“We believe that the medical aircraft was transporting a person off the island for additional medical care on the mainland,” said Crecy.Relatives of the missing person told US media outlets that the woman was being transported after having an allergic reaction. They also said her 20-year-old son went with her in the helicopter to help translate.Sheriff Robert Luna said at an afternoon press conference that authorities were not yet identifying victims by name. But he described the two people that died as the male pilot and a female nurse. He also said another medic and a fourth person, a man, were hospitalised.The helicopter’s operator, REACH Air Medical Services of Sacramento, confirmed that two of its crew members died and a third was recovering at a hospital. The company declined to provide more details.“In the days ahead, our focus will remain on caring for our people, supporting their families and honouring the lives and service of those we have lost,” REACH said in a statement. A memorial service was held on the island to honor the victims.The crash came just 11 days after a firefighting helicopter went down, killing two pilots in Yosemite National Park in California, and two weeks after a news helicopter crashed in Los Angeles, killing two others.First responders were called to the Catalina crash shortly before 8 pm local time on Wednesday and pulled four people from the water. Two were pronounced dead at the scene, and two, in stable condition, were brought to hospitals on the mainland, the Los Angeles County Fire Department said.The National Transportation Safety Board said it is investigating and identified the aircraft as a Eurocopter EC-135-P2.NTSB staff were expected to arrive on the scene later on Thursday to carry out the necessary physical inspections at the crash site prior to its removal. The aircraft will be brought to a secure facility for evaluation, a board spokesperson said.Authorities said the helicopter was submerged in about 70 metres of water off Catalina, which is located about some 35 kilometres off the mainland.The helicopter landed on the island shortly after 7 pm, following a 24-minute flight from an airport in Oceanside, according to the flight tracking platform Flightradar24.