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, Bradford, in the early hours of this morning.
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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."
Residents living close to the scene of the crash reported hearing a 'really loud bang' at about 5.30am.
Roger Wood, 67, said: "It woke me up really early in the morning, there was this really loud bang down the bottom of the road.
"It wasn't until I woke up properly later in the morning that I found out what had happened, when one of my neighbours said they'd seen on the news that people had died. It's horrendous, horrendous."
Another nearby resident, who did not wish to be named, said that she had long-held concerns about safety on Toller Lane.
The mother-of-two said: "I was horrified when I first heard about it, but I wasn't surprised when I heard that it was Toller Lane where it had happened.
"It's known for being a bit of a speed-trap and there have been accidents down there in the past."