An aerial cable car climbing into the high Alps this morning suddenly broke loose and plunged more than 300 feet to a steep mountainside, killing all 20 people aboard.

The cable car's driver and 19 passengers, many of them scientists and technicians, were heading to an astronomical observatory for their day's work when the high-altitude tram disconnected from its cable at about 7:15 a.m.

A spokeswoman for the local French prefecture said the victims died instantly on impact. An official inquiry was set in motion.

Michel Selaries, prosecutor for the Hautes-Alpes region, said criminal wrongdoing had been ruled out.

"Cable car accidents are like airplane crashes. They're very rare, but when they happen they are dramatic," said Jean-Charles Simiand, who runs a cable car company in nearby Grenoble and heads the French cable car operators' association.

The cable car and its mechanical system are the only means of access from the ski resort village of Saint-Etienne-en-Devoluy up to the Pic de Bur, a remote plateau nearly 9,000 feet high that is marked by six massive radio telescope dishes arrayed to face the heavens.

The cable car was reserved for professional use only -- ferrying scientists, technicians, contract builders or service staff to the Plateau de Bure Interferometer, showpiece of a French-German-Spanish state-funded astronomical consortium. Four of those aboard were with a French telecommunications installation company, Graniou Azur.

Early reports put the passenger list at 21. But Cyril Oddou, a mason scheduled to work on an observatory construction site, said he had failed to set his alarm in time to make the cable car departure.

The cable car system supporting this facility has only been in service 17 years; the whole system had a major overhaul only last year. Simiand said that the Plateau de Bure cable car was presumably in optimum condition, using the same service companies that maintain private sector cable cars. But because it does not carry members of the public, and despite the fact that it is state-owned, it is not subject to government safety and maintenance inspections.

The worst cable car accident occurred in 1976, in Cavalese, Italy, where 42 died. Last year's Italian cable car accident near Cavalese, in which 20 were killed, was caused by low-flying U.S. Marine Corps pilots.

The French Alps have been beset with accidents and natural disasters over the winter: avalanches in February killed 12, and a tunnel fire killed more than 45 in April. In this tourist-dependent ski country, cable car transportation is a way of life.