A man who killed a disabled neighbour in a vicious vigilante attack because he wrongly believed he was a paedophile will serve at least 18 years in prison for what the judge branded an "act of murderous injustice".

Lee James, 24, murdered Bijan Ebrahimi two days after police arrested the victim following unfounded complaints that he had a sexual interest in children.

Ebrahimi, a 44-year-old Iranian refugee described as gentle and harmless by his family, died after James repeatedly stamped on his head. James and another neighbour, Stephen Norley, 25, then dragged his body from his Bristol maisonette on to a triangle of grass, doused it with white spirit and set it alight. Norley was jailed for four years for assisting an offender.

Police have apologised for failing to protect Ebrahimi, who had been subjected to a campaign of harassment, and the police watchdog, the Independent Police Complaints Commission (IPCC), is investigating 11 officers and staff over the tragedy. Three officers who had dealings with Ebrahimi have been interviewed under criminal caution by IPCC investigators for the potential offence of misconduct in public office.

Nick Gargan, the chief constable of Avon and Somerset police, said the case was a source of huge regret, adding: "It is clear that there was a collective failure on the part of statutory agencies and others to protect Mr Ebrahimi. We need to have some frank and candid local discussions with our partners and our communities about what we collectively can do to stop this happening again."

Bristol city council has also launched an inquiry to find out if it could have done more to help Ebrahimi, who was housed by the local authority. The mayor, George Ferguson, called it an "exceptional crime" that did not represent the city.

The victim's family have said they feel let down by the authorities. His sister, Manizhah Moores, claimed he had long been subjected to abuse because of his race and his disability. She said he had come to the UK in 2000 aiming to make a better life for himself but had long been targeted because he appeared different from his neighbours.

Moores said: "The question that now must be answered is whether Bijan's death could have been avoided had he been afforded the protection from the authorities he deserved. Lessons must be learned before other vulnerable lives are lost."

Tony Murphy, the family's lawyer, said Ebrahimi had suffered "unimaginable pain" and was the victim of a hate crime. It emerged that just before he was killed, Ebrahimi had emailed police telling them he did not feel safe in his home.

Kerry McCarthy, the Bristol East MP, said issues that needed to be addressed included why the police did not make it clear that Ebrahimi was blameless after he was arrested and why he was left on an estate where he appeared not to be welcome.

Ebrahimi was called a paedophile after taking photographs of youngsters he suspected of vandalising hanging baskets at his home in Capgrave Crescent – and images of parents drinking in front of children, a practice he strongly disapproved of.

Bristol crown court was told that in the early evening of 11 July, there was a confrontation between Ebrahimi and James. Andrew Langdon QC, prosecuting, said: "Lee James was shouting at Mr Ebrahimi, telling him in abusive terms to stop taking pictures, to give him the camera, to delete the photographs."

James flagged down a police car driven by a community support officer and told him that Ebrahimi had been "looking at my kids". James said that if the police did not act he would deal with the situation himself. He said he was not scared of going to prison and would do anything to protect his children.

Ebrahimi was arrested by police on 12 July. When he was taken away, neighbours cheered. He was allowed to return home and was murdered in the early hours of 14 July.

Jailing James for life, Mr Justice Simon said he would serve a minimum term of 18 years before being considered for parole.

The IPCC said two police constables who attended Capgrave Crescent on the evening of 11 July, and another constable who was made aware of issues regarding Ebrahimi the following day, had been interviewed under criminal caution by IPCC investigators for the potential offence of misconduct in public office.

A sergeant and a constable who had contact with Ebrahimi in custody on 12 July have been interviewed for gross misconduct and remain subject to investigation. A control room supervisor, a dispatcher and four call handlers who dealt with phone calls from Ebrahimi have also been interviewed by the IPCC for gross misconduct.

In addition, investigators have carried out house-to-house inquiries, taken statements from members of the public and reviewed CCTV evidence from the custody suite at Broadbury Road police station where Ebrahimi was taken.

It has looked at contacts Ebrahimi had with the police going back "a number of years" and liaised with agencies including Bristol city council to look at issues such as housing and antisocial behaviour.

The IPCC commissioner Jan Williams said: "The conduct of a total of 11 officers and civilian police staff remains subject to our investigation. We have had ongoing dialogue with the Crown Prosecution Service and at the conclusion of our investigation I will be considering whether to send them a file of evidence in respect of any officers."

Sue Mountstevens, Avon and Somerset's police and crime commissioner, said the behaviour of James and Norley had been abominable.

She said: "I have critical questions for the police and other agencies about the events leading up to Mr Ebrahimi's death. I am determined to look publicly at what could or should have been done differently.

"I will be holding a public meeting involving the police, Bristol city council and any other organisations involved so that questions can be asked and any lessons learned in order to prevent anything like this from happening again."