The defence lawyer of a crown witness in a major gangland investigation was shot dead in the Amsterdam district of Buitenveldert on Wednesday morning, police say.

Derk Wiersum was representing Nabil B, a key witness in the case against gangland boss Ridouan Taghi. Taghi is said to be behind a string of killings in Utrecht and Amsterdam in the ongoing the ongoing ‘mocro mafia’ drugs wars.

The shooting happened around 7.30am. Neighbours reported hearing several shots and the gunman, said to be aged 16 to 20 and dressed in black, made off on foot.

Last year Nabil B’s brother was also shot dead, in what is thought to have been either a warning to Nabil not to testify or a case of mistaken identity.

Derk Wiersum was a partner in a major Amsterdam law firm and focused on cases involving organised crime. He had previously warned that the public prosecution department is not doing enough to protect crown witnesses.

Prime minister Mark Rutte and legal affairs minister Sander Dekker said they are extremely shocked by the killing and Amsterdam mayor Femke Halsema has called an emergency meeting of police chiefs and justice ministry officials. ‘This case has the highest priority,’ the mayor said.