In this case it was the video

From the Guardian: G20 death: 'This might have been swept under the rug' - eyewitness

As he sat down in his office in New York yesterday, the hedge fund manager who filmed the moment a police officer clashed violently with Ian Tomlinson digested the latest revelations.

He has chosen to remain anonymous but has been observing events from across the Atlantic. Informed by the Guardian that the second postmortem had found Mr Tomlinson died not of a heart attack but abdominal haemorrhage, he said he was relieved he had stepped forward as a witness.

"Judging by the short amount of time that lapsed between him being hit and pushed to the ground and him collapsing and dying, it just seemed to be coincidental that it was called a heart attack," he said.

"Now I'm glad I came forward. It's possible Mr Tomlinson's death would have been swept under the rug otherwise. There was nothing except some witnesses speaking to the Guardian saying they saw him being beaten. But it was their statements versus the police. You needed something incontrovertible. In this case it was the video."

THE LESSON OF THIS STORY: Where ever you are (London, New York, Burlington or any of the 'Constitution-Free-Zones' throughout America), if you every see anybody pulled over, harassed (or 'dog handled') by the oinks, stop and observe. And keep the camera rolling.

Photograph: Guardian.co.uk footage of Ian Tomlinson walking past a line of police dog handlers during the G20 protests in London.