A senior police officer who investigated the death of Ian Tomlinson told his family that the officer who struck him at the G20 demonstrations could have been a member of the public "dressed in police uniform", it emerged last night.

The City of London police investigator made the comment at an emergency meeting with Tomlinson's family and the Independent Police Complaints Commission on 8 April, hours after the Guardian released footage showing the attack on the 47-year-old newspaper vendor.

Tomlinson collapsed and died of internal bleeding shortly after being struck with a baton and pushed to the ground. He had been trying to find a way home through police cordons near the Bank of England on 1 April.

The footage prompted the IPCC to launch a full criminal inquiry.

A report on issues surrounding Tomlinson's death by Inquest, the group that assists the families of people who die in police custody, said yesterday that the suggestion he might have been attacked by an imposter gave the impression that the investigation was biased .

"The City of London police completely failed to persuade the Tomlinson family of its impartiality, not least when they were told by an investigating officer that he was not ruling out the possibility that the alleged assailant may be a member of the public dressed in police uniform," it said. A source present at the 8 April meeting said the senior investigator's comment was made after he was pressed on how the identity of the officer could be established from the video.

The investigator agreed that the man who struck Tomlinson was likely to have been a police officer, but could "not rule out" the possibility that he was a member of the public.

The family believed this theory was fantastical. The video of the attack clearly showed that the officer who struck Tomlinson, who has since been suspended from duty and questioned under caution for manslaughter, was surrounded by more than a dozen police officers. The source said that the investigator claimed one possibility was that a member of the public had stolen a Metropolitan police uniform and equipment from the back of a police van before initiating the attack.

Tomlinson's family also said this week's home affairs select committee report into policing of the G20 protests failed to properly identify the issues that may have contributed to his death. "I feel very upset by the report," said Julia Tomlinson, his widow. "It refers to Ian collapsing but does not mention the video evidence showing that he was struck."

She said her family was particularly frustrated that the inquiry appeared to focus on the inexperience of some officers, suggesting they should receive more training.