The family of a 21-year-old athlete left with a broken neck after he was detained by police 20 months ago have spoken publicly for the first time, complaining they are no clearer about what happened.

Julian Cole is in a vegetative state following his arrest outside a nightclub in Bedford in May 2013. His brother Claudius has condemned the lack of progress by the Independent Police Complaints Commission in its investigation into the incident, and said he has been inspired by US campaigners to speak out.

Six Bedfordshire police officers are under criminal investigation and have been questioned over potential offences of grievous bodily harm and misconduct in public office. They are believed to deny wrongdoing and have not been suspended from duty. Two door staff from the club have also been questioned over potential GBH.

Cole’s family say the IPCC has told them it believes at this stage that the police were responsible.

Claudius Cole said: “When the IPCC took over the investigation we hoped that we would learn the truth. Twenty months on, we feel that the IPCC investigation has ground to a halt, the police officers are not being held to account ​and we are left without answers while we care for Julian – whose life has been destroyed.”

Julian Cole before his arrest in 2013. Photograph: Family collect

Cole, a sports science student at Bedfordshire University, had a so-called hangman’s fracture, with experts saying considerable force was used on his neck while his head was pulled back. Investigators are considering if officers were responsible for breaking his neck and, if so, whether it amounts to an act of misconduct or criminality.

The family claim police initially tried to mislead them in the immediate aftermath. They say one officer told them Cole was talking after his arrest, but that this would be impossible with a severed spinal cord. Another allegedly said Cole was drunk. He had consumed alcohol but tests show he was not drunk and was under the drink-drive limit.

“Bedfordshire police tried to cover up what their officers did to Julian by alleging that Julian was conscious and ‘chatty’ in the back of the police van,” Claudius Cole added.

The Coles, who are black, say they have been inspired to campaign by black families in the US who complain of excessive police force. Claudius Cole said: “I think the police may have behaved the same here as in the US – it is the same type of behaviour, it hasn’t been as well publicised. Julian did not need six officers to pin him down – he is only 5ft 5in and was unarmed – excessive force used is just the same. There are questions to ask about whether there may be a​ racial element to how they treated Julian.​”

On Monday evening, the Cole family take part in a “solidarity event” at the House of Commons with families from Ferguson, Missouri, where violent protests took place after the fatal police shooting of Michael Brown last August.

Julian Cole was arrested on suspicion of a public order offence. The case was dropped because of his condition.

The incident happened when Cole returned to Elements nightclub to ask for a refund after he and friends were asked to leave. Door staff seized him before police took hold of him. CCTV captures some of the incident but not the part where Cole apparently lost consciousness. He reappears on CCTV handcuffed and being carried by officers into a police van.

Cole is now in a care home and his family are devastated. Claudius Cole said he had wanted Julian to be his best man last summer. Instead, pictures of his younger brother were projected at the wedding and family and friends went to visit him at the home.

The IPCC said: “Our investigators updated the Cole family just before Christmas, apologised for the length of time taken and gave reassurances over the latest timescales. Additional IPCC staff have now been allocated to this investigation to ensure it is completed as soon as possible.

“Six officers have been interviewed under criminal caution for misconduct in public office and grievous bodily harm. The IPCC also took over the criminal investigation into two door staff who were also interviewed under criminal caution for grievous bodily harm. A decision on whether to refer the case to the CPS will be taken upon completion of the report.”

In a statement, Bedfordshire police said: “We continue to follow a national syllabus and standards of training, all which are approved by the College of Policing; this includes the safe restraint and control of detainees.” It said it was awaiting the IPCC’s findings. “This was a complex set of circumstances involving a number of people. It would be inappropriate to apportion blame to any individual while there is an ongoing investigation.”