No police officer below the rank of commissioner or chief constable achieved a higher profile over the last half century than John Stalker, former deputy chief constable of Greater Manchester, who has died aged 79. First, as the person at the centre of the “Stalker affair” during the Northern Ireland Troubles and then as a broadcaster and writer, he was a key figure in the shaping of public attitudes to criminal justice and the police.

Born in Miles Platting, a suburb of Manchester, one of the four sons of Jack Stalker, an aircraft engineer, and his wife, Theresa, a machinist, John was educated at Chadderton grammar school and originally wanted to be a journalist.

After a job as a reporter with the Oldham Chronicle was not forthcoming, he decided to become a police cadet. Early in his career, he worked as a detective in Moss Side and promotion came swiftly. He was one of the officers involved in the Moors Murders investigations in the mid-1960s and the experience, notably listening to the grim tape-recordings made by the murderers, led him to say later that, while he was strongly opposed to capital punishment, he might have made an exception for Ian Brady and Myra Hindley.

He was an empathetic man whose sharp intelligence led to his appointment to the local Special Branch at a time, in the 70s, when the IRA was at its most active. While most of his career was based in Manchester, he was, for two years from 1978, head of the CID in Warwickshire, at the time the youngest chief superintendent in the country. In 1980 he was appointed assistant chief constable of Greater Manchester police force, and in 1983 was chosen to be part of a course at the Royal College of Defence Studies in connection with policing in the event of a nuclear war.

The following year he was promoted to deputy chief constable, tipped for high police office as a chief constable and even seen as a future Metropolitan police commissioner in the style of Sir Robert Mark.

Then came the role that was to define his career. In 1984 he was appointed as the officer in charge of the investigation into what was described as a “shoot-to-kill” policy allegedly carried out by some members of the security services in Northern Ireland. Stalker came from a family with Irish roots and his mother was a Roman Catholic; he could not, it was felt, be accused of taking the establishment’s side.

Stalker’s job was to find out exactly how six unarmed men had been shot during a brief period in 1982 and whether there was a deliberate policy of killing men believed to be IRA members rather than arresting and charging them. His appointment caused resentment among some Royal Ulster Constabulary officers in Belfast who felt that the English police understood little of the pressures and threats they were under. Sir John Hermon, the RUC’s chief constable, presciently warned Stalker that he had entered “a jungle”.

The key fatal shooting, for Stalker, was that of 17-year-old Michael Tighe, who had no paramilitary connections and had been shot in a hayshed in Lurgan in which an MI5 bug had been hidden; Stalker was desperate to retrieve this evidence but was constantly stalled.

Before he could complete his report, he was suspended in 1986 by his chief constable, James Anderton – himself no stranger to controversy – and subjected to a disciplinary investigation himself. Anderton later described his decision to suspend his deputy as “the blackest day of my life”.

At the centre of this investigation was Stalker’s friendship with a Mancunian businessman, Kevin Taylor, whom he had got to know as their daughters were schoolmates. Extraordinary efforts were made to discredit both Stalker and Taylor by linking them unsuccessfully to members of the Quality Street Gang, a bunch of local dodgy characters.

One of the gang, Jimmy “the Weed” Donnelly, who was asked by police for damaging information on the pair, later wrote in his memoirs that “they [the police] even asked the drag artist Foo Foo Lammar if Taylor or Stalker had ever been seen in any gay bars or dancing with other men. It was ridiculous.” Donnelly said he knew Stalker “about as well as I knew Elvis Presley”.

Stalker’s supporters – and there were many both within the police service and the media – cried foul. Was this a plot to derail an investigation that would have come up with embarrassing findings at a time when peace negotiations were under way? Or was he the victim of petty jealousies within the police service itself?

In the event, Stalker was finally cleared by the inquiry and his suspension lifted. But the affair took its toll on both him and his close-knit family and he decided to resign in 1987.

His bestselling autobiography, Stalker, was published in 1988 and he pulled few punches. He suggested that MI5 had promised to assist in his investigation and assured him that they would be “honest brokers” in his battles with the RUC.

“I do not doubt,” he wrote, “that my discovery of the existence of the MI5 tape of the killing of Tighe in the hayshed, and my pursuit of it, created very real anxiety. I was breaking new ground in my demands for access, and anti-terrorist operators within MI5 and the Special Branch were bitterly unhappy about even speaking to me.”

The book heralded the start of the career in journalism which might have begun three decades earlier.

He became a successful broadcaster, writer and public speaker, presenting Crimestalker for Central Television and Inside Crime for Carlton and writing opinion pieces for the national press. He did follow in the footsteps of Mark in that he used his reputation as a straight cop to make television commercials, for patio awnings and garage doors. He also acted as an adviser on security for Millwall football club.

He had a long and happy marriage to Stella Bendon, some of it spent on a smallholding in Cheshire. In 2006, after she had been attacked by two rottweilers, he campaigned for changes to the 1991 Dangerous Dogs Act.

Stella, whom he married in 1961, died in 2017. Stalker is survived by his daughters, Colette and Francine, six grandchildren and two great-grandchildren.

• John Stalker, police officer, writer and broadcaster, born 14 April 1939; died 15 February 2019