Pilot error was suspected in the crash of an Italian jetliner trying to land at Zurich's international airport, killing all 46 people aboard, including six Americans, authorities said Thursday.

The Alitalia DC-9, arriving on a 50-minute flight from Milan, skimmed treetops before its fiery crash into a wooded hillside on Wednesday night, five miles short of Kloten airport, police said.Josef Meier, a spokesman for Swiss air traffic control authorities, told a news conference that witness accounts suggested several causes for the crash and that no possible theories, including terrorism, were being ruled out.

But Meier said radar data "suggest the pilot misjudged his altitude." He said the plane was flying about 1,000 feet below the normal altitude during the landing approach.

He said the plane was not yet within reach of an airport radar beacon enabling it to lock onto a landing path.

Radio contact was normal until two minutes before the crash, and controllers received no distress signal, officials said.

The Swiss Federal Meteorological Office said visibility at the time of the crash was good - up to 10 miles, with light rain and light winds.

Investigators reported finding one of the two flight recorders, or "black boxes," which track flight data and cockpit conversations.