FLIMS, Switzerland - Twenty people died when a vintage World War II aircraft crashed into a Swiss mountain at the weekend, police said Sunday, as an investigation was launched into why the plane plummeted straight down at high speed.

"The police have the sad certainty that the 20 people aboard perished," police spokeswoman Anita Senti told a news conference.

There were 11 men and nine women aboard, including an Austrian couple and their son, she said.

The German-built Junker JU52 HB-HOT aircraft, dating from 1939 and nicknamed "Iron Annie", was a collectors' aircraft.

It crashed into Piz Segnas, a 3,000-meter (10,000-foot) peak in the east of the country on Saturday at an altitude of 2,540 meters on the mountain's western flank, Senti said.

According to German-language newspaper Blick, the flight had taken off from Ticino in the south of the country and had been due to land at the Duebendorf military airfield near Zurich on Saturday afternoon.

Swiss reports said the passengers were returning from Locarno, a holiday spot in southern Switzerland on Lake Maggiore, where they had arrived early Friday.

The 20 Minutes newspaper quoted a witness who was on the mountainside at the time of the crash.

"The plane turned 180 degrees to the south and fell to the ground like a stone," the witness said, adding that the debris was scattered over "a very small area" -- indicating that an explosion was unlikely to have been the cause of the crash.

NO BLACK BOX ABOARD

Grisons canton chief of police Andreas Tobler said there was "no longer any hope of finding anyone alive" among the 17 passengers and three crew members.

An investigation has been launched into the cause of the accident.

Twenty emergency workers were still at the crash site 24 hours after it happened, with helicopters hovering above for hours.

As this kind of collectors' aircraft is not equipped with "black box" flight data and voice recorders, investigators must rely on eyewitness accounts and analysis of debris.

Daniel Knech of the Swiss safety investigation service SESE said the crew did not have time to send out a distress signal. He said hot weather conditions did not contribute to the crash and that the aircraft fell almost vertically out of the sky at relatively high speed.

Knech also ruled out the idea that the plane had hit an object, such as a cable or another aircraft.

The aircraft belongs to JU-Air, a company with links to the Swiss air force, the ATS news agency reported.

JU-Air CEO Kurt Waldmeier told reporters that the plane had undergone a maintenance inspection in July.

He added that the flight had an experienced flight crew of two pilots and one assistant aboard, all in their 60s.

'DEEPLY SADDENED'

JU-Air said on its website that it was "deeply saddened" and its "thoughts were with the passengers, the crew and families and friends of the victims".

The company's flight operations were suspended, it said.

JU-Air says it runs a small fleet of 4 unker planes, all built in 1939, which are for hire. Its pilots are ex-military and professional pilots, all volunteers.

The Junker JU52 is made of corrugated steel and was built by the German firm Junkers from the 1930s to 1950s. It was used as a military transport plane as well as a bomber during World War II.

On its website, JU-Air mentions one past accident, in 1987, at the Koblenz airport in Germany in which no one was hurt.

The incident was the worst in Switzerland for several years, but just one of 3 plane crashes in the country in the space of 8 days.

In another Swiss plane crash on Saturday, a tourist plane carrying a couple and 2 young children crashed in a forest in the Nidwald canton and immediately burst into flames. No survivors have been found.

On July 27, 4 people were killed in a light plane crash at the top of a glacier in the southern Valais canton.