Three adults and a one-year-old boy have died in a collision with a car that had earlier been tailed by police.
Shortly after 20:50 GMT on Friday a VW Golf collided with a VW Touran people carrier turning right onto Bannham Road in the direction of Darnall, Sheffield.
The Golf had been followed by a police vehicle minutes before the crash.
Three other passengers in the Touran were also injured, including a three-year-old girl who was taken to hospital with life-threatening injuries.
Police watchdog investigates
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 men travelling in the Golf, aged 23, 17 and 18 suffered minor injuries. 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 in the Touran died later in hospital.
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.
‘Incredibly tragic accident’
“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.
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.