The killer of Rhys Jones was brought to justice by a high-risk strategy that pushed at the boundaries of what was permissible during a police investigation.

The name of the killer was “bandied about” in the gangland of north Liverpool. But to obtain evidence that would lead to a successful prosecution, police used bugging devices, the granting of immunity from prosecution and special measures to allow witnesses to give evidence from behind screens.

A surveillance team planted secret listening devices in the homes of defendants to eavesdrop on conversations. Even now officers refuse to divulge details of the operation. But the team listened to thousands of hours of material to isolate a few minutes of valuable information. This, along with mobile phone traffic, was examined to build up a picture of the interaction between participants and how the incident unfolded.

Within a week Detective Superintendent David Kelly saw enough to convince him of Sean Mercer’s guilt. But it was one thing to put him in the frame, and another to build a case that was sufficiently strong to put before a jury.

After a career spanning 30 years, the detective realised early on that this was going to be a complex inquiry and that “we were in for the long haul”.

“There was no doubt we had a mountain to climb,” he said. “But we were determined that nothing was going to get in the way of getting justice for Rhys.”

More than 200 officers were recruited to the inquiry; 60 specialist officers scoured the murder scene in 15-hour shifts for five days. More than 120 sq m of scrubland were stripped and tagged for laboratory examination. The search, however, threw up little valuable evidence.

Detectives made impassioned pleas for patience after the initial round of high-profile arrests. To the outside world it looked as if little progress was being made.

There was also the pressure that the team of detectives placed upon themselves. Mr Kelly recalls going to see Melanie Jones, 43, Rhys’s mother, at her home.

“She was clearly upset,” he said. “When I was leaving she got hold of my hand and looked into my eyes. They were almost pleading. I knew then, more than at any time, the enormous size of the task that I faced.”

For Mr Kelly, the breakthrough came on September 30 last year, when officers raided a suburban home in Croxteth and discovered a .455 Smith & Wesson revolver in the attic. The teenage gun minder, identified only as “Boy C”, was arrested on his return from a holiday in Florida.

The transformation of Boy C, a teenager on the outer fringes of the gang, from a potential suspect to the chief prosecution witness was the turning point of the investigation.

The possibility of granting immunity under the Serious Organised Crime and Police Act 2005 sent Crown Prosecution Service lawyers scurrying for their law books. It was only the second time that the legislation had been used in earnest, and the first for a youth.

Boy C’s detailed testimony, effectively placing the gun in Mercer’s hand, led directly to “strike day” in April. Another 200 officers were brought in to execute 11 warrants in dawn raids across north Liverpool, leading to the arrest of 13 suspects and eventually the seven defendants in the dock.

Boy C and his family can never return to their home and remain under protection.

Throughout, Mr Kelly has been acutely aware of his responsibility to Rhys’s parents. Mrs Jones and her husband, Stephen, had acted with dignity and courage, he said.

“They had the biggest right to be the most demanding of me, yet they demonstrated patience more than anybody,” Mr Kelly said. “They did not put me under any undue pressure. They knew we had to go carefully because we did not want to make any mistakes that could undermine the investigation. That is some pressure to carry.

“You have heard [Melanie] say that they would not be able to rest or move on until those responsible were brought to justice. I just hope that we have achieved that for them . . . and that they can find some rest, albeit that they will never forget Rhys. It has been an honour to work on this case and to try to bring the killer to justice.”

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