CAVALESE, Italy (Reuters) - A U.S. military warplane that struck cable car lines in northeast Italy, sending 20 skiers plunging to their deaths, had probably been flying under the cables, an Italian police commissioner said on Wednesday. "We suppose the plane went under the cables some 100 metres (325 feet) off the ground," said Paolo Sartori, police commissioner in Trento, some 50 km (30 miles) from the accident site. He said police had found a piece of the EA-6B Grumman Prowler, a four-man surveillance aircraft, about 100 metres (yards) from the point where the cable car crashed to the ground. Sartori, who is part of a group of Italian investigators leading an inquiry into how the plane could have sheared the cable lines in clear weather conditions, said the aircraft`s four crew members would be questioned later on Wednesday. Investigators from the U.S. air base in Aviano in northeast Italy, where the plane landed safely after Tuesday`s tragedy, were also reportedly heading to Cavalese to start their own investigation. "We are waiting for the U.S. commission to help us understand the situation better," Sartori said. The 20 German, Belgian, Italian, Austrian, Polish and Dutch nationals were killed after returning from a day`s skiing on the Cermis slopes when their cabin fell 200 metres (650 feet) into the mountainside.