An inquiry has been launched by the police watchdog following the death of a man who was restrained using pepper spray.
Jacob Michael, 25, was arrested for alleged affray in Widnes, Cheshire, and taken by police van to a custody suite.
The spray is understood to have been used on him during the course of the arrest on Monday evening.
Shortly after he arrived at Runcorn police station in Cheshire he became unwell and paramedics were called, the Independent Police Complaints Commission (IPCC) said.
He was taken to Warrington General Hospital by ambulance, where he was pronounced dead.
A spokesman for the IPPC, which is investigating his death, said: "At about 5.15pm on Monday, two police officers attended an address in Lacey Street in Widnes in response to a call received.
"Following a call for assistance, further officers arrived and the 25-year-old man was arrested for alleged affray.
"During the course of the arrest it is understood police deployed pepper spray."
A post-mortem examination carried out yesterday failed to establish the cause of the young man's death and further tests will be conducted "as a matter of urgency to identify any contributory factors", the IPCC spokesman said.
Investigators spent yesterday gathering evidence and initial accounts from police officers, and the police van Michael was transported in was secured for examination, along with CCTV from the custody area, he added.
IPCC Commissioner Naseem Malik, who is overseeing the probe, said: "Investigators have been in Widnes collecting evidence that has helped us begin to start to piece together what happened when police attended Jacob's home.
"Aside from the information we have already gathered, I am aware that people have made comments on social media sites and to journalists indicating they may have witnessed police detaining Jacob on Lacey St. I would ask those people to contact our investigators on 0161 246 8511.
"A cause of death has not been established yet and we have asked that further tests be conducted with the utmost urgency. While we await these answers, I ask people to remain calm and have patience. I can assure them this will be a thorough investigation, the findings of which will be aired and tested in a public forum at some point in the future."
Cheshire Police said today that post-mortem results showed "no physical injuries on Michael that could be attributable to a cause of his death".
In a statement, Assistant Chief Constable Philip Thompson said: "Whilst pepper spray was discharged during the course of his arrest, there is no evidence that the use of pepper spray was the sole factor, or indeed a contributory factor, to Mr Michael becoming unwell some time after his arrest, or as a cause of his death.
"These are amongst the reasons why further extensive tests will be required to establish an exact cause of death.
"The police are co-operating openly with the IPCC and have provided detailed briefings to local councillors and other partners concerning the event.
"The family of Mr Michael are being supported by liaison officers and the IPCC is ensuring they are being informed of the progress of their investigation.
"I appeal for people to remain calm to allow the IPCC to conduct their investigation effectively and swiftly and await the outcome of the investigation before making any judgment on this matter."