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BERLIN (AP) _ German air traffic controllers said Monday they tried to warn a Swiss control tower by phone two minutes before two planes collided, killing all 71 people on board, but the only available line was busy.

German controllers in the southern city of Karlsruhe made the call to the Zurich tower _ which was in charge of directing the planes _ after receiving an automatic radar warning that a cargo plane and a Russian charter jet were on course to crash over southern Germany, Axel Raab, a spokesman for the Karlsruhe control center, told The Associated Press.

Investigators are focusing on the actions of Swiss air traffic controllers and whether they gave the Bashkirian Airlines Tu-154 enough time to descend before it collided with the DHL International Boeing 757 just before midnight July 1 at 35,000 feet.

Under the division of airspace along the German-Swiss border, the planes were being directed by controllers in Zurich, Switzerland. Swiss prosecutors have opened a criminal investigation to determined whether anyone can be charged with negligent homicide.

German investigators said last week that the telephone system at the Zurich control center was being worked on at the time of the crash and that the lone controller on duty was working on a reserve phone line. In addition, the center's collision-warning system was out of service of maintenance.

Investigators say the Zurich tower was making phone calls to the airport in Friedrichshafen, Germany, to coordinate another flight _ the last one 98 second before he gave a first warning to the Russian plane. But Joerg Schoenberg, the lead German investigator at the crash site on the shores of Lake Constance bordering Switzerland, said this did not play a role in the accident.

All 69 people aboard the Russian aircraft, including 45 students headed for a Spanish beach vacation, and the two DHL pilots were killed.

Most of the wreckage has been recovered and brought to the Friedrichshafen airport for investigation. A German lab in Braunschweig is examining the flight data and cockpit voice recorders of both planes.