New update:

According to Dutch newspapers the police 'witnessed' the assasination of Stanley Hillis. Police knew that Hillis had an appointment with Donny Groen at that time. So they hide themselves in somekind of vehicle. From there they wanted to listen to Hillis what he had to say against the other criminal. The vehicle was just 2 metres from the place Hillis was shot down.

Hillis was shot down before Donny Groen arrived, who arrived too late on his appointment. Groen was arrested immediately, but the criminals who actually shot Hillis got away. Even a helicopter couldn't track them down.