The police cannot be held responsible for a mass shooting at a shopping centre in the Netherlands in 2011, despite making mistakes in giving a licence to the gunman, a court in The Hague said on Wednesday.

Relatives of the six people who were killed in the shooting in Alphen aan den Rijn hope to make claims against the police, who, they say, should never have given a gun licence to killer Tristan van der Vlis.

The court ruled that the police had made errors in their assessment of Van der Vlis and that he should not have been licenced to own weapons. He had been refused one in 2005, but this was not taken into account in the new application. Nor was the fact he was forcibly detained in a psychiatric hospital in 2006, the court said.

Nevertheless, the police cannot be held responsible for Van der Vlis’ violent actions, the court said in a statement.

A spokesman for the relatives told broadcaster Nos the verdict is ‘cowardly’ and said they would appeal. The relatives say the police are jointly responsible for the shooting because they knew Van der Vlis had psychiatric problems but still gave him a licence to own weapons.

Van der Vlis used a semi-automatic weapon to kill six people and injure 17 before turning the gun on himself in April 2011.

The official investigation into the shooting spree revealed that Van der Vlis was probably a paranoid schizophrenic who suffered repeated periods of depression. He also made at least two suicide attempts in 2008.