Turkish media say a policeman has been stabbed to death in Istanbul by a suspected Islamist militant who was in custody.
The state news agency Anadolu said the aggressor had been driven to police headquarters and was being taken to his cell when he struck.
He was immediately shot dead by the security forces.
The man suspected of links to the so-called Islamic State had been detained because he was suspected of preparing an attack.
The officer, named as Sinan Acar, was taken away by ambulance but died in hospital. Despite a temporary media blackout over the attack a crowd gathered outside.