Four young friends died after a car hit a tree as it was being followed by police.
The men were in a BMW which crashed on Toller Lane, Bradford, in the early hours of yesterday morning.
The Independent Office for Police Conduct has begun an investigation following the crash.
A spokeswoman for the office said: "It is understood at this time that the car was being followed by an unmarked police car at the time of the crash."
A manager of a nearby coffee shop described those who died as "polite young men".
Mohammed Rashid (51) said that the people involved in the crash had been going to his shop since they were boys and were all friends. He added: "It's an absolute tragedy. Your heart just goes out to the families, to their parents. They were all polite young men, although I only knew them to greet and say hello to every now and then when they came in."