New and disturbing evidence of systematic and nationwide attempts by police to smear anti-racism campaigners surfaced last night, as the Independent Police Complaints Commission (IPCC) mounted an investigation into the “improper surveillance” of the grieving sister of a black paratrooper who died a slow and agonising death in custody.

A letter to Janet Alder, seen by The Independent, reveals that Humberside Police has discovered evidence that both she and a lawyer who helped prove the force’s shocking failure to prevent her brother’s death in April 1998 were illegally monitored by officers at the time of his inquest in 2000, which returned a verdict of unlawful killing. The Independent can also reveal that the former West Yorkshire Police Chief Constable, Sir Norman Bettison, commissioned a six-page report into Mohammed Amran, a Bradford race relations worker, before he testified to the Macpherson inquiry into the death of the black teenager Stephen Lawrence in 1998.

That evidence led the IPCC to announce yesterday that it will launch an investigation to examine whether Sir Norman was “motivated or influenced by racial discrimination”. Mr Amran’s case was first revealed by this newspaper three weeks ago.

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The IPCC has told Ms Alder that she appeared to be “the subject of improper surveillance during the course of the inquest following the tragic death of your brother” and that the organisation “has determined that an independent investigation should be carried out to establish what surveillance took place and the reasons for it.”

The development is potentially deeply damaging for Humberside Police, 15 years after Christopher Alder’s death – face down, in handcuffs, his trousers down around his thighs, on a Hull police station floor – became one of the most controversial in police custody. CCTV footage showed the father of two children gasping for air as officers chatted and joked around him.

His sister’s indignation initially stemmed from the fact that it took the force 14 days to inform her of his death. She felt that police were monitoring her as soon as she began asking probing questions – concerns which led her local Burnley Labour MP Peter Pike to register concerns about surveillance with the former Police Complaints Authority, a predecessor to the IPCC.

“I had plain clothes officers following me around shops and on one occasion I left out of the back door of a solicitors’ office because I thought I was being followed,” Ms Alder said yesterday. “I also had mail that went missing. They characterised me as being paranoid and over-dramatising things. This evidence only confirms what I always believed. I want to know why they were doing it and on whose authority.”

The name of the lawyer whom the force also subjected to surveillance is known to The Independent, but he does not want his identity to be revealed while he and his advisers seek a response to the force’s evidence. The IPCC will also investigate whether any other individuals were spied upon by the force at the time of Mr Alder’s inquest.

Ms Alder also has documentary evidence which suggests that police attempted to secure sensitive personal documents about her, during her long campaign, supported by the Lawrence family, to prove that her brother’s death was avoidable and to persuade the Crown Prosecution Service to bring manslaughter charges against five police officers.

Humberside Police attempted to secure social services documents which related to the period during which she and her brother were in care, as children. This evidence was not used during the inquest or the manslaughter trial, which collapsed in 2002 when a judge ruled there was no evidence to convict.

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“The police wanted to go into our family history,” Ms Alder said. “They were told that kind of information is only appropriate if there is a threat to the country or if there is a criminal inquiry.”

Ms Alder said she also expected the referral of information to disprove the IPCC’s own 2006 report into her brother’s case, commissioned by David Blunkett, who was the Home Secretary at the time, which dismissed her claims that she had been followed.

Humberside Police said last night it had undertaken an internal review in response to “interest in covert policing tactics” and had “chosen to refer to the IPCC an operation conducted a number of years ago”. The force said it could offer no further comment.

Mr Alder’s body was discovered in the Hull Royal Infirmary mortuary in November 2011, 11 years after his family believed they had laid him to rest. An exhumation of his grave in Hull’s Northern Cemetery early last year confirmed Grace Kamara, 77, had been buried in his place.

“The evidence of the surveillance is deeply significant because Janet has always thought she has been treated in the most extraordinary way by Humberside Police,” said her solicitor, Ruth Bundey.

Police monitoring: Under surveillance

The Metropolitan Police

Ex-undercover officer Peter Francis alleged that the Lawrence family and supporters were put under scrutiny. The watchdog said the allegations were “serious and indicate potential grave misconduct”.

West Yorkshire

Sir Norman Bettison is to be investigated over a dossier compiled on anti-racism campaigner Mohammed Amran, over suspicions he was “influenced by racial discrimination”.

Humberside

Police have discovered evidence that the family and lawyer of Christopher Alder were put under surveillance.

Manchester

The force’s Special Branch asked in 1998 for details of people likely to attend Macpherson inquiry hearings in the city but the police watchdog said it appeared routine requests for information were sent out.

South Yorkshire

South Yorkshire called for monitoring of “left-wing extremists” at events peripheral to the Macpherson hearings but the watchdog said there was no evidence to link it to the Lawrence family or supporters.