Ueberlingen, Germany ? Swiss prosecutors opened a criminal investigation Thursday into the collision of two airliners as grieving families of dozens of children killed in the crash piled flowers around the aircraft wreckage.

Meanwhile, initial results of a German-led inquiry into the crash which killed 71 people, 45 of them Russian children headed for an end-of-school beach vacation found a Russian pilot had been given just 44 seconds warning before slamming into an oncoming cargo plane.

The investigations turned fresh attention on Swiss air traffic control, which took charge of the planes shortly before the crash.

Swiss prosecutors said their investigation was opened amid suspicions of negligent homicide. The aim is to establish whether any actions by Swiss air traffic control could prompt criminal charges, said Christoph Naef, a spokesman for Zurich prosecutors.

Already, the Swiss have said there was only one controller in the Zurich tower at the time and there should have been two because a crash avoidance system was out of service for maintenance. The second controller had taken a break.

Chief German investigator Peter Schlegel said analysis of radio transmissions showed the Bashkirian Airlines Tu-154 was given six seconds less than the 50-second warning Swiss and German officials had previously reported. That is nearly a minute less than the Swiss had claimed shortly after the crash.

The Russian-made Tupolev began to dive just 14 seconds after the initial warning and 30 seconds before the crash. The Russian plane was responding to a second Swiss warning.

“The Tupolev should have begun descending at the latest 1 1/2 minutes before the crossing point,” Schlegel said, at a news conference in the north central German city of Braunschweig.

But he said it was too early to determine blame for the collision with a Boeing 757 DHL International delivery service jet just before midnight Monday. Both pilots on that plane died.

Schlegel denied a Russian report that experts decoding the flight data found the Russian pilot asked to change course 1 1/2 minutes before the collision.