The Netherlands is taking legal action in an effort to find out more about the 2012 bus crash in a Swiss tunnel in which six Dutch children died, the AD said on Friday.

The Swiss authorities have refused to give the Dutch authorities a blood sample taken from the driver amid claims he may have deliberately crashed the bus while under the influence of anti-depressants.

Relatives of those killed in the crash are convinced it was no accident and that Geert Michiels drove the bus in the wall of the tunnel on purpose.

The Swiss authorities say there is no reason to hand over the sample because the investigation has been completed.

‘We are now going through legal channels to try to answer the questions that parents have been left with as well as we can,’ a foreign affairs spokesman said.

The cause of the crash, in which 28 people died, has never been fully explained. The official conclusion is that it was a freak accident caused by the driver fainting or through negligence.