A police force is facing questions over its treatment of disabled people, after two reports by the independent watchdog into the way its officers dealt with young autistic men.

In both reports, the Independent Police Complaints Commission (IPCC) investigation was impeded because crucial video evidence had been erased by Northamptonshire Police.

Last year, the force admitted that it had launched an internal review of the way it treats disabled people, in the wake of the two incidents, despite originally denying that it was taking place.

It has refused to say this week if that review has been completed.

In the first case, an officer was caught on mobile phone footage apparently punching an autistic 17-year-old several times and swearing at him, after he and a trainee police constable had responded to reports of a disturbance at a house, in August 2015.

The two officers had been asked by the owner of the house to remove the 17-year-old, who apparently refused to let the officers search him.

By the time IPCC investigators launched their investigation, the officer had “accidentally” deleted his own footage of the incident that had been captured by his body-worn video camera, after showing it to his supervisor.

Despite the IPCC investigator concluding that the officer should face disciplinary charges of gross misconduct, Northamptonshire Police and an IPCC commissioner decided these charges should be downgraded to misconduct.

The officer was later given a written warning for “failing to treat the teenager with respect and courtesy by using inappropriate language, failing to properly record use of force”, and deleting his camera footage.

The officer’s supervisor was also disciplined after it emerged that he had viewed the footage but took no steps to ask the officer about his use of force, or to save the footage.

An edited version of the mobile phone footage of the incident, which took place in the bedroom of a friend of the 17-year-old, was subsequently shared widely on social media and in the mainstream media.

The Crown Prosecution Service (CPS) decided there was insufficient evidence to bring any prosecution against the officer for alleged excessive force.

Disability News Service has also been passed an IPCC report into a second set of allegations against the force.

This report investigated concerns about the way Northamptonshire Police dealt with a young autistic man who was held in a cell for eight hours, after he had defended himself from a vicious disability hate crime in a local park in Rushden, in October 2015.

Daniel Smith was only finally able to clear his name after a six-month ordeal which saw him dragged through the criminal justice system by the police and CPS, before he was eventually cleared of assault charges by magistrates.

He had been left bloodied and bruised after the attack but ended up being prosecuted for assault after the police refused to investigate the hate crime and charged him instead.

He spent eight hours in the cell, without medical treatment for his injuries, even though he told officers he had only been defending himself against “the bullies”.

But despite Daniel’s father, Owen, informing the force eight days later that he wanted to complain about the incident, the custody sergeant at the centre of the allegations was not interviewed by the IPCC until February 2017, 16 months later.

And because the force refused to accept the complaint until Owen obtained written consent from his son – who by that time had returned to his home in Devon – CCTV footage of what took place in the police station had been recorded over by the time the complaint was accepted.

An IPCC spokeswoman said the case had only been referred to them by Northamptonshire Police in June 2016, after the end of Daniel Smith’s trial.

When asked why the custody sergeant who had dealt with Daniel was not interviewed until February 2017, a further seven months later, she said: “Investigators spent the initial stages of the investigation gathering relevant evidence, including interviewing Daniel, to assist with their interview of the custody sergeant.

“CCTV from custody suites is automatically recorded over after 30 days as a national standard, unless a request is made to secure it, for example, when a formal complaint is made.”

Daniel’s father insists that his son told officers that he had Asperger’s syndrome, but the IPCC report concludes that “none of them recall” him telling them of any impairment.

The IPCC investigator concluded that there was “insufficient evidence” to justify the custody sergeant facing a misconduct hearing.

IPCC had failed to say by noon today (Thursday) whether it believed the force should have taken steps to preserve the CCTV footage.

When asked whether it was just a coincidence that vital footage that might have been used to prove a case against officers from the force was deleted in both of the investigations, an IPCC spokeswoman said: “There is no evidence to suggest the two incidents are related in any way.”

She said that the force’s internal review of its treatment of disabled people was “a matter for Northamptonshire Police”.

And asked if the two cases showed the force had a problem with dealing with autistic people, she said: “We have conducted two thorough investigations looking at each complaint individually.

“In both of these cases, there is insufficient evidence to suggest officers involved were aware of the complainant’s autism.”

Owen Smith is also angry that neither the force nor IPCC have examined the failure to treat the attack on his son as a disability hate crime.

The IPCC spokeswoman said this was not part of the IPCC investigation and that Northants police carried out its own investigation into the incident that led to Daniel’s arrest.

Northamptonshire Police had not responded to requests for a comment by noon today (Thursday), despite being asked for a response on Monday.