Twelve people have been arrested as part of the investigation into the A66 crash that killed seven people, including two police officers.
Nine men and three women, aged between 19 and 61, have been arrested across Teesside on suspicion of a range of offences including possession of a firearm and being part of an organised crime group. One has since been charged.
In the early hours of Saturday, a Volkswagen Passat that had been pursued by police travelled the wrong way down the A66 near Middlesbrough and crashed into an oncoming police car - two PCs and all five Passat occupants died.
Cleveland Police said it was investigating "serious criminal activity that resulted in this fatal incident".
PC Tom Clough, 38, and PC Matthew Blades, 37, died in the crash, as did Michael Robert Cahill, 23, Jacob Matusiak, 23, Makai Saddington, 18, Theo Rae, 17, and 17-year-old Cole Robert Worthy.
Earlier, Cleveland Police released more details of its investigation into the lead-up to the crash and information about the 12 people it had arrested.
The force said:
It received several reports of two vehicles "being driven dangerously" on Saturday morning
The first vehicle, believed to be a Volvo XC60, was seen "ramming properties" in the Middlesbrough area
The second vehicle, a Passat, is believed to have been associated with this activity
Both vehicles were operating with cloned number plates but were picked up by automated number plate recognition technology
The Volvo has been recovered in Middlesbrough
Four of the five people in the Passat had previous convictions
Assistant Chief Constable Wayne Fox said the investigation had been "fast moving and complex".
He said: "The events of that tragic night will have a lasting and profound impact on the Cleveland area and the communities we seek to protect.
"We have a duty to respond to, and thoroughly investigate, the serious criminal activity that resulted in this fatal incident and to bring all those responsible to justice."
The Independent Office for Police Conduct (IOPC) has launched an investigation into the collision following a self-referral from Cleveland Police, which is mandatory when someone dies following contact with police.
The IOPC said officers had followed the Passat for almost seven minutes before abandoning the pursuit when the car went the wrong way down the A66 at South Bank.
Ten seconds later at 03:39 BST, the Passat collided with the vehicle of PCs Clough and Blades, who were travelling on the correct side of the road and were in the area to assist with the search for the car.
Earlier, BBC Verify reported that a TikTok account believed to belong to Saddington previously shared videos showing young men driving dangerously.
The government called for social media platforms to remove posts celebrating such behaviour.
Cleveland Police said: "At the current time, there is no evidence to indicate that filming of the activities that took place on Friday night and Saturday morning has taken place for the purposes of TikTok."
An online fundraiser set up for the officers' families by the Cleveland Police Federation raised more than £750,000 by Tuesday morning.
PC Blades' wife said he "gave his life in the line of duty, protecting and serving others with courage, dedication and selflessness".
Before joining Cleveland Police, PC Blades worked as an English teacher at Carmel College in Darlington.
The college said: "Many within our community will remember Matty as an intelligent, kind and approachable teacher, a valued colleague and someone who gave generously of himself to the young people in his care."
The family of PC Clough described the former soldier as "our hero".
They said he "jumped into action whenever needed" and was "willing to help anyone for any reason".
The family statement added that he "proudly fought for his country to make the world a better place".
Flags at Cleveland Police headquarters are flying at half-mast and Redcar and Cleveland Council has opened a book of condolence at its offices on Ridley Street in Redcar.
Floral tributes can also be left at the memorial garden at the force's headquarters in Hemlington or any Cleveland Police station, the council said.
Tributes for the five young men have also been shared on social media, with many of their friends saying they were "heartbroken" and "lost for words". Balloon releases have been announced in their memory.
