LONDON, England (CNN) -- A man who collapsed and died near last week's protests at the G-20 summit was shoved to the ground by police shortly before the collapse, according to a video of the incident posted on the Web site of the British daily The Guardian.

A video posted by the Guardian shows police shoving Ian Tomlinson to the ground. He later died of a heart attack.

A spokeswoman for the Independent Police Complaints Commission, which is already investigating any contact Ian Tomlinson had with police before his death, said she is certain the man in the video is Tomlinson, 47, who was pronounced dead on arrival at a hospital after his collapse on April 1.

"However, that is not the question," the spokeswoman said. "The question now is that we see someone getting beaten up by police, and we have to investigate it."

The commission said it was seeking to obtain the video and "will be assessing this along with the other statements and photographs that have already been submitted." Watch video showing police allegedly shoving man »

An autopsy revealed that a heart attack caused Tomlinson's death, according to the police complaints commission.

In the video posted on The Guardian's site, the man identified as Tomlinson can be seen stepping in front of a line of police officers with dogs, his hands in his pockets, and walking slowly. After a few seconds, an officer without a dog, but wielding a baton, rushes him from behind and knocks him to the pavement.

The line of officers stops and a civilian approaches Tomlinson. For a few moments, Tomlinson appears to speak with the officers. The camera then pans the line of officers, and Tomlinson can be seen walking across the frame away from the officers.

Shortly after the video was shot, Tomlinson collapsed.

The police complaints commission announced Monday that it was conducting an investigation into one event before Tomlinson's death and had reviewed several hours of footage from the closed circuit television network that covers much of London.

In his first appearance on the CCTV footage, Tomlinson, who is believed to have been walking home from work, approaches a police cordon opposite the Bank of England, where some of the protests were taking place. Officers refused to let him through, the commission said.

Next, Tomlinson shows up on the video a few blocks away. Witnesses recalled seeing him in that location, where he was caught up in a crowd and pushed back by police officers, the commission said.

The commission was investigating that incident.

"This would have been a few minutes before he collapsed," said Deborah Glass, commissioner for London. "It is important that we are able to establish as far as possible whether that contact had anything to do with his death."

A short time later, the commission said, CCTV cameras captured Tomlinson leaving that area.

"Mr. Tomlinson walks for about three more minutes, before collapsing," the commission said.

Bystanders tried to help him, while others alerted police. Officers arrived, bringing with them emergency services personnel. They loaded Tomlinson on a stretcher and transported him to the hospital, where he was pronounced dead.

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