Derk Wiersum was the lawyer for Nabil Bakkali, a crown witness in a case against 16 men accused of five murders.

A Dutch lawyer in a high-profile case against a group of men charged with a series of drug-related killings was shot dead in Amsterdam on Wednesday morning, police said.

The victim was identified as 44-year-old Derk Wiersum, a defence lawyer for Nabil Bakkali, a crown witness in the case against 16 men accused of five murders and one attempted murder in the Netherlands between 2015 and 2017.

The killing of Wiersum, a father and long-time criminal lawyer in Amsterdam, has sent shockwaves through the city and its justice system.

Police said they were searching for a youth aged between 16 and 20, who ran away after the shooting, which took place shortly after 7:30am local time (05:30 GMT).

Bakkali is among those accused in the case, which is likely to go to trial next year, but has since 2017 provided police with over 1,500 pages of statements against the group in return for a lower sentence, preliminary court proceedings have shown.

His brother, who police say had nothing to do with the gang or the murders, was killed at his workplace last year, a week after justice authorities said Bakkali was their crown witness.

The prime suspect in the case is 41-year-old Ridouan Taghi, who remains at large. Through his lawyer, Taghi has denied all accusations.

Other suspects, 11 of whom are in custody, have remained silent throughout the case.

Femke Halsema, the mayor of Amsterdam, called the killing an “unprecedented attack on the legal profession that affects the essence of the rule of law”.

“This is a horrible murder, of a father, a lawyer, an Amsterdammer … This leads to fear and unrest among lawyers. For the police and the public prosecution, the case has the highest priority.

“The Amsterdam triangle [police, prosecution and local government] is meeting today to discuss the latest state of affairs … I sympathise with everyone who has been affected in any way by this terrible act. I wish the family strength with the processing of this indescribable sorrow.”