Rakhat Aliyev, who held several posts in Astana government, was due to stand trial in Vienna when he was found hanging in his cell

The Austrian justice department has rejected suggestions of murder in a long-running controversy over the death in jail of Rakhat Aliyev, once one of the most powerful figures in Kazakhstan until he fell foul of his former father-in-law and president of the country, Nursultan Nazarbayev.

Aliyev, 52, a former ambassador to Austria, intelligence officer, deputy foreign minister and banker, who had expressed fear that his life was in danger from Kazakhstan’s KNB secret police, was found hanging in his cell in Vienna’s Josefstadt prison in February, prompting a series of conspiracy theories. He had been awaiting trial on charges of kidnapping and murdering two Kazakh bankers.

The prison authorities immediately pronounced his death as suicide but lawyers acting on behalf of his second wife, Elnara Shorazova, challenged this. But in letters sent out last week and obtained by the Guardian, an Austrian judge has dismissed the challenges and concluded Aliyev hanged himself. “The objection on the grounds of infringement of law by Elnara Shorazova is rejected,” the judge wrote.

The ruling almost certainly brings an end to a saga that offered glimpses of into the murky side of life in the oil-rich former Soviet republic. The events surrounding Aliyev’s death have attracted huge media attention around the world.

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In two separate letters posted on 9 and 11 December, the judge, Maria Schörghubel, acknowledged that the case had been unusual. There was an investigation into the possibility of foul play “because of the nearness of his trial, his particular exposure, threats made against him in the past while he was in custody, and the potential involvement of foreign powers”, she wrote. But in the end she supported a police conclusion that it had been suicide.

Lawyers acting for Shorazova had submitted a detailed list of questions to the justice department, including an independent pathology report that concluded that “the involvement of a third party cannot be excluded”, that “at least two people are needed to stage such a hanging”, and a seemingly unexplained bruise in the middle of his forehead.

The lawyers expressed concern that the “crime scene” had not been secured and that evidence had potentially been lost, in particular the failure to establish the time of death. They questioned too the physical feasibility of the hanging and noted what they claimed to be two mystery footprints on the wall next to the scene. They questioned why a knife visible on CCTV footage had apparently not been submitted in evidence.

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Schörghubel did not accept that the knife was pertinent to the case, accepting testimony of prison guards that it had been used to cut the body down. She quoted one of the staff, Franz Berger, as saying: “I went to the bathroom, [saw the body], checked the neck to see whether there was a pulse, and didn’t find one. His skin was cold. I ran out to a work room 30 metres away, fetched a knife.”

In a second complaint, Shorazova said two postmortems on Aliyev failed to establish a time of death because the scene had been compromised. One was carried out immediately after. The judge said further tests were carried out in August by another expert, who found traces of benzodiazepine and zolpidem, a sleeping tablet, in Aliyev’s blood. But the judge said doctors had been giving Aliyev sleeping pills in his cell, with two tablets dished out every day. There was no indication that the doses went beyond usual levels.

The judge concluded that the fact that no exact time of death could be determined was not materially relevant to the investigation, since there was insufficient evidence to suggest he had been murdered.

Aliyev had been a surgeon when in the 1980s he married Dariga, daughter of Kazakhstan’s authoritarian leader. He was appointed to several key government posts, including deputy head of intelligence. Helped by his political connections, he built up a business empire in the oil-rich republic. He fell out with the president in 2007, was stripped off all government posts and was divorced. He became a vocal critic of the president.

Just before his death, Aliyev had been due to testify in the trial of two prisoners who he alleged had threatened to kill him and make it look like a suicide unless he paid them.