Seven people, including two police officers, have died in a crash following a pursuit in the early hours of Saturday.
A Volkswagen Passat was travelling the wrong way down the A66 near Middlesbrough and collided with a police vehicle on the same road going in the right direction, Cleveland Police said.
Two serving police officers in the marked vehicle and five members of the public travelling in the Passat were pronounced dead at the scene after the crash at 3.39am on Saturday.
Police are still working to formally identify all occupants of the Volkswagen Passat but have informed the families of those identified and offered them specialist support.
The Independent Office for Police Conduct (IOPC) opened an investigation into the circumstances leading to the crash in South Bank after a mandatory referral from the force.
Officers from Cleveland Police spotted a suspicious vehicle and a pursuit began before the car travelled onto the A66 and collided with a police car, the watchdog said.
The A66 is closed in both directions between Normanby Road and Church Lane in Eston.
Police officers manned both ends of the cordon and the crash site was not visible from the road closures.
A large white tent has been erected over the scene, with multiple emergency service vehicles around.
Police have put into effect a drone exclusion zone around the scene of the collision.
The closed section of the A66 is bordered by large industrial units to the north and is close to the major Teesworks site.
Chief Constable Victoria Fuller, of Cleveland Police, said the force would co-operate fully with the investigation.
“While this investigation takes place, we strongly urge people to avoid speculation,” she said.
Prime Minister Andy Burnham said he was “devastated” to hear seven people were killed in the collision.
Home Secretary Shabana Mahmood said the crash was “devastating”, adding her thoughts were with all those affected.
Anna Turley, MP for the Redcar constituency where the crash happened, said the A66 would remain closed in both directions between Normanby Road and Church Lane, and urged the public to avoid the area and use alternative routes.
She said: “Cleveland Police have confirmed that seven people have sadly lost their lives.
“Their families and loved ones are now facing an unimaginable loss that will be felt forever.
“Two of the seven people who have lost their lives were Cleveland Police officers, on duty serving our community.
“They went to work to keep people safe and they should have come home to their families.
“Their loss will be felt deeply by their loved ones, fellow officers and right across the communities they served.”
David Taylor and Stephen Martin, councillors for the Eston ward, said they were “deeply saddened” by the “unimaginable tragedy”.
A post on their Facebook page read: “We are deeply saddened by the devastating news that seven people have lost their lives following a collision on the A66 at South Bank in the early hours of this morning.”
It continued: “This is an unimaginable tragedy that will be felt deeply throughout TS6 and communities across Cleveland and around the country.
“Absolutely heartbreaking.”
Cleveland Police Federation chairwoman Lauren Somerville said: “Words cannot describe the pain the Cleveland Police family are feeling today.
“Our colleagues who should have been going home to their loved ones at the end of their shifts are not. And that is just unfathomable.
“Cleveland Police Federation will be offering full support to our fallen colleagues’ families and they have all of our love and care at this time.
“When the time is right we will pay full tribute to our brave colleagues. But we can assure that their sacrifice will never be forgotten.”
