A Sweden-bound ferry that sank Tuesday, killing 1,102 people, had a door missing in its bow, according to Swedish authorities. Investigators said the door may have been ripped out by crashing waves, causing the ship to roll over and sink in a matter of minutes off the southwestern coast of Sweden. Another possibility is that the door "may have been left untied," says TIME reporter Ulla Plon in Copenhagen. Authorities maintained that the ferry complied with Swedish maritime safety standards. "They are now looking at whether those safety standards should be changed," says Plon. Investigators are using an unmanned, undersea robot to look for clues on the ferry, which lies at a depth of about 250 ft.