BERLIN (AP) - A bus carrying German senior citizens on vacation crashed into a truck Monday on a highway in Bavaria and burst into flames, killing 18 people and injuring 30 others, some seriously, officials said.

The accident took place around 7 a.m. when the bus rear-ended a trailer-truck at the end of a traffic jam on the A9 highway near Muenchberg, not far from the Czech border. The accident led to long traffic jams on the A9, the main thoroughfare from Berlin to Munich.

It was not immediately clear what caused the crash, but prosecutors have opened an investigation.

Police experts work on the burned out shell of the travel bus at the place of the accident on the motorway A9. near Muenchberg, Germany, Monday, July 3, 2017. Several people are feared dead after a bus carrying a group of German senior citizens crashed into a truck on a highway in Bavaria early Monday and burst into flames, police said. (Nicolas Armer/dpa via AP)

The bus seemed to have caught fire immediately upon impact with the truck, Transport Minister Alexander Dobrindt told reporters after touring the crash site. The truck's trailer also burst into flames.

"Two people are still in life-threatening condition," Dobrint said.

Two drivers and 46 passengers were on the bus. The remains of all bodies were recovered from the bus, including that of the driver at the wheel during the crash, police said.

Police said the truck driver was not injured.

Some 200 emergency crews rushed to the scene and five helicopters whisked the injured to nearby hospitals. Simple wooden coffins were wheeled in for the remains recovered from the blackened, twisted wreckage of the bus.

Police said the ages of those on the bus ranged from 41 to 81 and they were primarily from the eastern German state of Saxony. The news agency dpa reported that all the passengers were German.

German Chancellor Angela Merkel thanked the emergency crews involved.

"Our thoughts are with the families of the victims and we wish all those who were injured a speedy recovery," she said.

Dobrindt said when firefighters first arrived, the fire was so hot that they couldn't get anywhere close to the burning vehicle to rescue those stuck inside.

"All they could do was extinguish the fire," he said. "The heat was so strong that nothing that's inflammable is left from the bus - only the steel frame."

The A9 highway remained closed on both sides for hours. Police tweeted later that the A9 highway going south would be shut down for the day.

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David Rising contributed to this story.

Police and rescuers stand near coffins on the motorway A9 near Muenchberg, Germany, Monday, July 3, 2017. Several people are feared dead after a bus carrying a group of German senior citizens crashed into a truck on a highway in Bavaria early Monday and burst into flames, police said. (Nicolas Amer/dpa via AP)

Rescue workers stand beside a burnt-out coach on the motorway A9 near Muenchberg, southeastern Germany, Monday, July 3, 2017 after a bus crashed into a truck and caught fire. (News5 / Fricke/NEWS5/dpa via AP)

Fire fighters and rescuers stand beside a burnt-out coach on the motorway A9 near Muenchberg, southeastern Germany, Monday, July 3, 2017 after a bus crashed into a truck and caught fire. (Nicolas Armer/dpa via AP)

Undertakers bring two wooden coffins to the place of the accident on the motorway A9 near Muenchberg, southeastern Germany, Monday, July 3, 2017 after a bus crashed into a truck and caught fire. (Nicolas Amer/dpa via AP)

Rescue workers stand beside burnt-out coach on the motorway A9 near Muenchberg, southeastern Germany, Monday, July 3, 2017 after a bus crashed into a truck and caught fire. . (News5 / Fricke/NEWS5/dpa via AP)

Rescue helicopters sit on the motorway A9 near Muenchberg, southeastern Germany, Monday, July 3, 2017 after a bus crashed into a truck and caught fire. Leaving several people dead. (News5 / Fricke/NEWS5/dpa via AP)

In this photo taken from a video smoke rises from a coach after a crash on the motorway AP near Muenchberg, southeastern Germany. Police say that people might have been killed in the crash. (News5/ via AP) GERMANY OUT

Vehicles stand in a traffic jam on the motorway A9 near Muenchberg, Germany, Monday, July 3, 2017 after a bus crashed into a truck and caught fire. (Nicolas Armer/dpa via AP)

Fire fighters stand beside a burnt-out coach on the motorway A9 near Muenchberg, southeastern Germany, Monday, July 3, 2017 after a bus crashed into a truck and caught fire. (Nicolas Armer/dpa via AP)

Rescue workers and fire fighters stand beside a burnt-out coach on the motorway A9 near Muenchberg, southeastern Germany, Monday, July 3, 2017 after a bus crashed into a truck and caught fire. (Nicolas Armer/dpa via AP)

Rescue workers and firefighters are seen on the motorway A9 near Muenchberg, southeastern Germany, Monday, July 3, 2017 after a bus crashed into a truck and caught fire. (News5 / Fricke/NEWS5/dpa via AP)

Police and rescuers stand on the motorway A9 near Muenchberg, Germany, Monday, July 3, 2017. Several people are feared dead after a bus carrying a group of German senior citizens crashed into a truck on a highway in Bavaria early Monday and burst into flames, police said. (Nicolas Amer/dpa via AP)

An aerial view shows the place of the accident on the motorway A9 near Muenchberg, Germany, Monday July 3, 2017 after a bus crashed into a truck and caught fire. Several people are feared dead after a bus carrying a group of German senior citizens crashed into a truck on a highway in Bavaria early Monday and burst into flames, police said. ( Bodo Schackow/dpa via AP)