Witnesses say Mr Tomlinson collapsed after earlier contact with police
The Independent Police Complaints Commission (IPCC) is to oversee an investigation into the death of a man during protests before the G20 summit.
Ian Tomlinson, 47, died of a heart attack near to clashes between police and protesters in London on 1 April.
Witnesses have told the IPCC that the newsagent, who was not part of the protest, had "contact" with police officers before collapsing.
City of London Police will carry out the inquiry on behalf of the watchdog.
Units of riot police and other public order officers were involved in a complicated operation to quell sporadic bursts of minor violence near the Bank of England throughout Wednesday.
The vast majority of the demonstrators were peaceful but thousands of people became hemmed in by police at Royal Exchange Square, opposite the Bank of England.
IPCC Commissioner for London Deborah Glass said CCTV footage showed Mr Tomlinson, who was returning to his home in the area after work, collapsing in nearby Cornhill before being given first aid by police medics.
Image raises questions over death
She said: "Initially we had accounts from independent witnesses who were on Cornhill, who told us that there had been no contact between the police and Mr Tomlinson when he collapsed.
"However, other witnesses who saw him in the Royal Exchange area have since told us that Mr Tomlinson did have contact with police officers.
"This would have been a few minutes before he collapsed. It is important that we are able to establish as far as possible whether that contact had anything to do with his death."
The IPCC said its decision to manage the probe will be kept under review.
A number of witnesses have described seeing him there, getting caught up in a crowd and being pushed back by police officers
Deborah Glass, IPCC Commissioner
Ms Glass appealed to witnesses who saw Mr Tomlinson at about 1920 BST or who may have taken a photo of him around that time to get in contact.
After Mr Tomlinson's death was referred to the IPCC on 1 April, investigators viewed CCTV footage, examined statements and police records and spoke to independent witnesses.
The IPCC said CCTV shows police officers refused to let Mr Tomlinson through a cordon opposite the Bank of England, prompting him to walk around the corner into Royal Exchange Passage.
"A number of witnesses have described seeing him there, getting caught up in a crowd and being pushed back by police officers," she added.
"This is the aspect of the incident that the IPCC is now investigating."
The IPCC said CCTV footage taken later of Mr Tomlinson walking back onto Cornhill before collapsing, does not show him trapped inside a police cordon.
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