Three adults and a one-year-old boy have died in a crash with a car that had been tailed by police.
A VW Golf collided with a VW Touran people carrier turning right on to Bannham Road towards Darnall, Sheffield, on Friday night.
The Golf had been followed by police minutes before the crash.
Three passengers in the Touran were also injured, including a three-year-old girl who was taken to hospital with life-threatening injuries.
A 22-year-old woman from the Touran is in a critical condition and a second woman from the Touran is in a serious but stable condition.
Three males travelling in the Golf, aged 23, 17 and 18, suffered minor injuries in the crash, which happened shortly after 20:50 GMT.
They have been arrested and remain in custody.
From the Touran, two men aged 35 and 50, one of whom was the driver, and a 41-year-old woman, all died at the scene.
A one-year-old boy who had also been in the Touran died later in hospital.
'Incredibly tragic accident'
Two families are believed to have been travelling in the people carrier, South Yorkshire Police said.
The police watchdog, the Independent Office for Police Conduct (IOPC), has started an investigation.
Regional director Miranda Biddle said: "As soon as we were informed, we sent investigators to the scene and we will be carrying out an independent investigation to establish what actions South Yorkshire Police took prior to the crash.
"This is standard procedure when lives are lost following an incident involving the police."
"There is no indication that the police vehicle collided with either of the vehicles involved," she added.
The IOPC said it had spoken to the officers involved and taken statements.
At the scene - Corinne Wheatley - BBC Look North
Police investigating this crash have described it as horrific and have, as far as possible, kept people away from the wreckages of the two vehicles.
Officers have thanked the local community for their response, bringing the emergency services tea and water, but have urged people to remain respectful and mindful of the material they share on social media.
Det Ch Insp Jude Ashmore, of South Yorkshire Police, said he was thinking of the victims' families following the "incredibly tragic accident".
"Our priority now is to support the victims' families and those affected, and work hard to determine the exact circumstances around what happened," he added.