Activists are calling for a new public investigation into the death of an asylum seeker who burnt to death under mysterious circumstances in a police cell in Dessau, east Germany in 2005.

Police had previously claimed that Oury Jalloh, originally from Sierra Leone, set fire to his own mattress and died before firefighters could come to his rescue.

A police officer was sentenced to a €10,800 fine for involuntary manslaughter in 2012 after he admitted to ignoring the fire alarm.

But a new report commissioned by family and friends of Jalloh suggests that the 36-year-old may not have died by accident, but was instead murdered. Compiled by an arson investigator from Ireland, the document shows that the kind of burns found inside the cell could only have been caused by the use of additional fire accelerants.

The people who commissioned the report have filed a legal complaint with the federal public prosecutor and are now hoping for a new trial at a national court. Nadine Saeed, spokesperson for the initiative In Memory of Oury Jalloh, says the new findings disprove the theory that Jalloh could have set the fire himself.

Many questions in the case remain unanswered. Dessau police claimed that Jalloh set fire to his mattress even though his hands were tied down on either side and despite the fact that no lighter was found on him when strip-searched.

A melted-down lighter was only discovered three days after the death; an analysis showed that it contained neither traces of the mattress nor of Jalloh's clothes or body.

It took an independent autopsy paid for by Jalloh's friends and relatives to show that he had suffered a broken nose and ruptured ear drums. The police autopsy had failed to note this.

The police have in the past claimed that Jalloh was arrested under the influence of alcohol and cocaine after molesting a group of women outside a nightclub. After acting aggressively, he was locked in a cell, his hands and feet tied. Later that evening, they claimed, he set fire to his mattress with a lighter and died of heat shock.

Jalloh's friends and relatives have repeatedly questioned this version of events. How could a man, lying on his back with hands tied to the ground on both sides, have set fire to a mattress with an anti-inflammable coating, they ask. How was he able to hide the lighter behind his back afterwards? And how come no one spotted the lighter when he was strip-searched earlier that evening?

In 2008, a trial against two police officers ended inconclusively due to lack of evidence. A second trial in 2012 led to one of the officers, Andreas S, being handed a €10,800 fine for involuntary manslaughter. He admitted having turned down the intercom and switched off the fire alarm when it went off.

But on Tuesday, members of the In Memory of Oury Jalloh initiative presented new evidence, which they say disproves the theory that Jalloh could have set the fire himself. The report suggests that the kind of burns found on the site of Jalloh's death could only have been caused by the use of additional fire accelerants such as petrol or lighter fluid. Maksim Smirnou, a certified arson investigator based in Ireland, has conducted a series of tests recreating the conditions inside the cell.

The chances of Jalloh's death gaining new attention in Germany may be higher now than when the story first made the headlines. The much-publicised trial against the far-right National Socialist Underground has revealed that authorities had not only failed to adequately investigate the neo-Nazi gang implicated in the murder of nine migrants and a policewoman, but also in some instances actively hindered their arrest.

In the Jalloh case, too, there are allegations that authorities attempted to mislead investigations or cover up the affair entirely. The lighter found under Jalloh's body was not on the original list of items found at the crime scene.

A video of the burnt-out cell filmed two hours after the end of the fire abruptly cuts off after just four minutes – supposedly because of a power cut – and thus doesn't show whether the lighter really was under the dead man's body. An analysis of the melted lighter has shown that it contained neither traces of the mattress nor of Jalloh's clothes or body.

In 2007 a leak revealed that the head of policing in Dessau was trying to slow down the work of three officers looking into crimes with a rightwing motive, supposedly because it was damaging to the reputation of the region.

Saxony-Anhalt, where Dessau is located, has the highest rate of violent crimes with a rightwing motive in Germany, despite the fact that it also has the lowest percentage of foreign-born residents (1.9%).

Speaking to the Guardian, one of the officers in charge of investigating police misconduct in Dessau described a conspiracy of silence among police officers: "There were certainly problems with cop culture on the Dessau beat," said Swen Ennullat.

He questioned whether the issue has been addressed adequately since he left Saxony-Anhalt in 2008: "So many of the people involved have since claimed that they have 'forgotten' what happened that day. I can't believe that you could forget a day like that."

After the latest report, parallels between the NSU trial and Jalloh's death are likely to attract international attention. Iyiola Solanke, a professor at Leeds Law School who focuses on racial integration, said: "Taken together, the cover-up of the NSU scandal and the superficial investigation into the death of Oury Jalloh raise questions about the conduct of German police towards black and minority ethnic victims of crime. The parallels are worrying and it would be hasty to brush them aside as mere coincidence."