A small private plane carrying three people and a New York tourist helicopter with six collided in midair and plunged into the Hudson River off Hoboken, N.J., opposite Manhattan's West Side, just before noon on Saturday. All on board the two aircraft were killed, the authorities said. The cause of the accident was under investigation. But what perhaps thousands of people out on a crystalline summer day saw from both sides of the Hudson was a stunning, low-altitude accident in which the plane rolled up and into the helicopter, striking with a crack like thunder as the helicopter's blades and one of the plane's wings flew off, and then both aircraft fell and vanished into the river. As witnesses watched from parks and balconies, abuzz with disaster, three bodies were recovered, one floating in the water and two others from wreckage believed to be that of the helicopter, located by divers on the murky riverbed 30 feet down. The search for the plane was impeded by visibility of only 2 or 3 feet at the bottom. But officials held out no hope for survivors. 'This is not going to have a happy ending,' Mayor Michael R. Bloomberg said at a news conference, contrasting the outcome with the spectacular landing of a jetliner in the icy Hudson on Jan. 15, when all 155 on board were rescued by ferries and emergency boats as they clung to the wings. From all appearances, the mayor said in somber tones, the crash was 'not survivable.' He said divers searching for the remaining victims, and then trying to recover the submerged aircraft, would proceed with caution to avoid further loss of life. 'This has changed from a rescue to a recovery mission,' he said. The victims included five Italian tourists and a pilot aboard the helicopter, which had just taken off from the West 30th Street heliport in Manhattan. Aviation authorities identified the pilot and owner of the plane as Steven Altman, of Ambler, Pa., and said he carried two passengers; a law enforcement official identified them as Altman's brother Daniel Altman and Daniel's son Brett. A dozen relatives of the Italian tourists, who were traveling together, were taken to a Red Cross center on West 49th Street, where they received counseling by chaplains and other assistance. The authorities did not identify the tourists who died. 