The Leipzig public prosecutor's office said on Thursday that it had opened an involuntary manslaughter investigation into the driver of the Flixbus that crashed on Wednesday, killing four.

The deadly incident occurred in the eastern state of Saxony when the privately run bus veered off the A9 highway and fell onto its side.

Four people were killed in the crash, six were seriously injured and another 29 were left with minor injuries.

What we know about the investigation so far

The bus, which was traveling from Berlin to the Swiss city of Zurich, had two drivers so that they could take turns.

A 62-year-old Czech man was driving at the time of the crash and is the subject of the investigation into manslaughter. The other driver was a 53-year-old Slovakian man.

Both were injured and the public prosecutor's office was not able to say whether the driver had been questioned yet.

The Flixbus company said that the driver had complied with driving and rest time rules.

"There were two drivers on board, the driver on duty had been driving the bus since its departure from Berlin at 8 am," it added.

Police said that the investigation will take some time as evidence still needs to be evaluated and witnesses questioned, some of whom are still being treated in hospital.

German newspaper the Leipziger Volkszeitung cited a witness who said that the journey had had a rocky start with the driver taking the wrong route. She also told the paper that the two drivers had been arguing loudly.

Police did not comment on this report.

Police reveal victims' identities

Authorities revealed the identities of three of the four deceased on Thursday, all of whom were women.

They included a 47-year-old Polish woman, a 20-year-old Indonesian woman residing in Berlin and a 19-year-old woman from Bavaria.

Police said the identity of the fourth victim was not yet beyond doubt.

The bus was carrying 54 people at the time of the crash, only 18 of whom were German nationals. The others came from more than 20 countries, including Peru, New Zealand, China, Canada and the United States.

Flixbus had initially said that 55 people were on board. This left authorities to announce a death toll of five as they searched for the missing person, presumed dead.

ab/wd (dpa, AFP)