BERLIN (AP) — Prosecutors in Germany say a bus driver's inattentiveness led to a collision with a truck on a highway last month that killed 18 people.

The bus carrying German seniors on vacation to Italy slammed into the back of a truck that had slowed for a traffic jam in Bavaria on July 3. The bus was quickly engulfed in flames and only a charred frame remained.

Prosecutor Jochen Goetz said Wednesday that the bus drove into the truck at 60-70 kph (37-43.5 mph). It wasn't clear what if anything distracted the driver, who died.

The impact damaged a part of the bus that contained batteries and a fuel tank. Goetz said the damage caused short-circuiting and hot electric arcs, and ruptured the tank — meaning that the fuel immediately caught fire.