Footage showed Mr Tomlinson being shoved by an officer - video courtesy Guardian.co.uk
The police officer thought to be shown in video footage of a G20 protest in London pushing a man who later died has come forward.
An independent criminal probe has begun into the death of Ian Tomlinson, 47, who had a heart attack minutes later.
Additional video footage from 1 April shown on Channel 4 News appears to show an officer striking him with a baton.
The Independent Police Complaints Commission (IPCC) is trying to identify officers who were at the scene.
The IPCC has ordered a second post-mortem examination as part of its investigation.
Mr Tomlinson's stepson Paul King said the footage has "deeply touched" the family and said "we cannot lay our father to rest" until the relatives get justice.
The IPCC began its investigation before footage surfaced of Mr Tomlinson being shoved.
Ian Tomlinson lived and worked in the City of London
The IPCC said it would examine the footage from 1 April and discuss it with the Crown Prosecution Service.
Following Mr Tomlinson's death the IPCC had initially said it would oversee an investigation into the incident to be carried out by the City of London police.
Deborah Glass, of the IPCC, explained the decision to take over the investigation: "People are rightly concerned about this tragic death and this footage is clearly disturbing.
"In light of new information which we became aware of yesterday (Tuesday) evening, we have now taken the decision to independently investigate.
"People have been calling for a criminal investigation. I want to stress that, from the outset of all our investigations, we consider whether criminal offences have been committed.
"This is a criminal investigation and we will, of course, be discussing it with the Crown Prosecution Service.
"At the moment the investigation is focused on identifying the officers in the footage. Several have already come forward and all efforts are being made to trace those who haven't."
Ms Glass appealed for more people to submit any other video footage of the incident.
The video, shot at 1929 BST at the Royal Exchange Passage on 1 April, initially shows Mr Tomlinson, who was going home from work and not protesting, walking away from a group of police officers.
Deborah Glass, IPCC: 'We continue to appeal for more information'
The footage, recorded by a New York fund manager, shows Mr Tomlinson receiving a two-handed push from an officer, landing heavily before remonstrating with the police.
Minutes later, Mr Tomlinson collapsed and died of a heart attack, after walking to nearby Cornhill where he received first aid from police.
Earlier the Metropolitan Police Commissioner Sir Paul Stephenson said the images raised "obvious concerns" and pledged full support from the Met.
The Guardian newspaper obtained the video and has handed it to the IPCC.
Opposition MPs have called for a criminal investigation into the death.
The police have well-established powers to use reasonable force if they think there is a threat either to themselves or the public, but these are enhanced during a protest or riot.
The key concept is that of "reasonable force" - i.e. force that is in proportion to the threat faced either by the public, the police or property Thus "reasonable force" may literally range from putting a hand on someone's elbow, to shooting them dead The legislation governing police powers during demonstrations (mostly the Public Order Act 1986) must also be seen in context of human rights' legislation Under European human rights' laws the police are required to actively protect the public's right to peaceful protest Policing during a protest, therefore, is a negotiation between the rights of police to use reasonable force to protect the public, and their responsibility to allow peaceful protest to take place
LOCATION OF IAN TOMLINSON AND G20 PROTESTS IN CITY OF LONDON
1. Protests at Bank Junction outside Bank of England
2. Protest outside RBS branch in Threadneedle Street 3. Climate Change tent city on Bishopsgate