The Guardian obtained this footage of Ian Tomlinson at a G20 protest in London shortly before he died. It shows Tomlinson, who was not part of the demonstration, being assaulted from behind and pushed to the ground by baton-wielding police guardian.co.uk

The police officer seen on a video by millions of people assaulting a man at the G20 protests minutes before he died will be questioned by investigators today after coming forward last night.

The territorial support group officer identified himself to his manager and the Independent Police Complaints Commission as fresh pictures suggested he had removed his shoulder number and covered his face with a balaclava before hitting Ian Tomlinson with a baton and pushing him to the ground last week.

But the officer has not been arrested on suspicion of assault or suspended from duty by the Metropolitan police.

He came forward along with several other police officers shown in the video obtained by the Guardian after a day of developments which saw:

• The IPCC reverse its decision to allow City of London police officers to investigate the death.

• A second postmortem examination ordered as the IPCC confirmed it would conduct a criminal investigation.

• Several top-level crisis meetings inside Scotland Yard.

• The Crown Prosecution Service liaising with the IPCC over possible charges.

• The emergence of Channel 4 footage taken from another angle which clearly shows the riot squad officer hitting Mr Tomlinson with his baton before pushing him to the ground.

The officer involved could face a manslaughter charge if a link is proved between the assault and Tomlinson's fatal heart attack minutes later in Cornhill in the City of London. "There is no excuse for what he did," a senior police source told the Guardian, adding that at the very least he had committed a serious disciplinary offence and a criminal assault.

Scotland Yard said yesterday four Met officers had come forward including one who had told his team leader that he was potentially involved in the assault.

Mr Tomlinson was on his way home at about 7.20pm last Wednesday when he was hit with a baton and pushed to the ground by a riot officer near the Bank of England. Witnesses suggest the attack, which was caught on film, was at least the second time police assaulted him.

At first the City police investigating the death said only that he died of natural causes, but the video footage forced the IPCC yesterday to review their role in the inquiry. Deborah Glass, senior commissioner, said yesterday that as a result of the footage the watchdog was removing the investigation from City police and taking it over as a totally independent inquiry.

"People are rightly concerned about this tragic death and this footage is clearly disturbing," she said as she announced that she had ordered a second postmortem on Mr Tomlinson. The first found he had died of a heart attack, but neither the police nor the IPCC would reveal whether any marks suggesting an assault had been found on his body. The family thanked the media for their help. Jules Carey, their solicitor, said they now requested "that their privacy be respected while they come to terms with the tragic loss of Ian".

There had been intense pressure on the IPCC to use its powers to conduct an independent investigation after the Guardian footage was released on Monday. Brian Paddick, a former deputy assistant commissioner at the Met, had demanded that police be removed from the case and said any officer who struck the innocent passerby could face a manslaughter charge.

He said the officers from the City of London force would be key witnesses in the investigation. "How can the City of London do the investigation independently?" he asked. "I'm sorry but there are three City of London officers in that video, how can they do the investigation?

"It certainly needs to be a full-blown criminal investigation ... [into] whether there is a provable link between the death and assault, because an assault is a criminal offence."

Sir Paul Stephenson, the Met commissioner, said the images "raise obvious concerns" and his force fully supported an inquiry. But he stopped short of suspending the officer involved pending the completion of the IPCC investigation.

Earlier Nick Hardwick, the IPCC chairman, demanded the officer seen in the footage first striking Tomlinson on the leg with a baton, and then shoving him from behind, should give himself up. He made clear all officers "are required by the new standard of conduct for police officers to report any wrongdoing". "If that officer has an explanation it must be in his interest to come forward," he said.

A source close to the IPCC said the issues for the investigation were: "Was an offence committed. Secondly did it contribute to his later death. Would that incident have caused the heart attack?"

Jacqui Smith, the home secretary, said it was important the IPCC inquiry was completed as soon as possible, adding: "If it identifies the need for a criminal investigation then that also needs to be pursued."