At around 5.30am on Thursday, a grey BMW 1 Series crashed on Toller Lane, Bradford.
The IOPC (Independent Office for Police Conduct) said West Yorkshire Police had begun a high speed pursuit before the BMW crashed into a tree.
All four young men died at the scene. Police have not yet named the men involved.
A spokesman for the IOPC said: "We understand that, prior to the incident, an unmarked West Yorkshire Police car began a pursuit of the BMW as it was being driven at speed.
"A short time later the car collided with a tree and all four men died at the scene."Two of the men have been formally identified. The other two men who died have not yet been formally identified.
"We have made the decision not to name the men at this time, until all their identifications have been confirmed and we are confident their next of kin have been informed.
-> VIDEO: Footage from scene of Bradford crash where four men died shows extent of wreckage"Our investigators attended the post incident procedures and will attend the post mortems of the four men when these take place. Statements from the officers involved, who are all being treated as witnesses, have been taken. "We have overseen the removal of the police vehicle involved, and the BMW, and these will now be forensically examined for use in our independent investigation.
"We have established that there is dashcam footage from the police vehicle, which will be analysed in due course. We are also in the process of gathering local CCTV footage and will be conducting house to house enquiries over the coming days. "Anyone who witnessed, or recorded, the incident is asked to contact us on 0800 096 9079 or email [email protected] "
-> Four killed in tragic Bradford crashIOPC Regional Director Miranda Biddle: “This was a horrific incident, resulting in the deaths of four men, and our thoughts remain with their loved ones and all those affected. “Our investigators responded quickly following the referral from West Yorkshire Police, and were at the scene shortly after the crash – this is vital to ensure we establish key evidence at an early stage. “I would like to take this opportunity to invite anyone with information about the crash to come forward.”