Four people have died in a car which was being followed by an unmarked police vehicle when it crashed.
The Independent Office for Police Conduct (IOPC) said it has been called in to investigate the crash, which involved a BMW, on Bingley Road in Bradford in the early hours of Thursday morning.
An IOPC spokeswoman said: “It is understood at this time that the car was being followed by an unmarked police car at the time of the crash.”
IOPC regional director Miranda Biddle said: “This is a most tragic incident and our sympathies are with the family and friends of those who have died.
“West Yorkshire Police made a mandatory referral to the IOPC and at 6.24am we declared an independent investigation.
“Our investigators are attending post-incident procedures, have travelled to the scene and have begun gathering evidence.”
West Yorkshire Police said four males died in the crash.
A spokesman said: “Due to the nature of the incident it has been referred to the Independent Office for Police Conduct.”
He said: “Police can now confirm that four males who were in a car which was involved in a road traffic collision at 5.30am on Bingley Road at the junction with Toller Lane in Bradford this morning have died.
“The road is currently closed and is likely to remain so throughout most of the day.”
Witnesses are reported to have described a loud bang and coming out to see a large amount of police activity with a number of cars involved in the crash.