Sean Mercer did not flinch when he saw a young child fall injured to the ground. Instead he carefully took aim, arms outstretched, and pulled the trigger for his his third and final shot.

The senior detective who brought him to justice believes that the 16-year-old hooded gunman is a cold-hearted killer, impervious to the moral consequences of his actions and marking him out as a very dangerous young man.

On his home turf in north Liverpool, he was known as "Beaver" and had already earned himself a squalid reputation for "mouthy" beligerence, hooliganism and casual violence as the pinched-faced wannabe gangster of the Croxteth Young Guns.

The teenager in his hooded North Face jacket and Nike camouflage T-shirt already had one anti-social behaviour order to his name for low-level gang activity, but few believed this would curtail his criminality. He was a ubiquitous sight on his mountain bike in the streets around the Dog and Gun pub. He was yet another teenager it was not worth crossing.

David Kelly, the officer in charge of the Rhys Jones murder inquiry, said: "Three shots were fired. It was the second shot that struck Rhys. Mercer had known that and he said later that a kid went down. Despite that he fired a third shot and that is significant to me. He knew what he had done and he knew he had hit and injured Rhys Jones yet he still aimed and fired again".

Mercer ignored numerous appeals, not least those from Rhys' mother and father. Instead, said Mr Kelly, he sought to distance himself from his crime, drawing in his close associates to help him to destroy evidence.

"That, in itself, provides some indication of the individual we are dealing with," said Mr Kelly. "It is not somebody who was subsequently remorseful or has shown remorse at any time during our dealings with him".

Sean Mercer was brought up by his mother Janette, 48, along with his sister and younger brother in a maisonette opposite De la Salle School, Croxteth, where he was a pupil. His father was a well-known figure on the doors of Liverpool's nightclubs.

Mr Kelly says he was, "just a kid hanging around the streets with other kids" but in his early teens began mixing with older members of the Croxteth Crew at a time when their turf war with the neighbouring Nogga Dogz was at its height.

By August last year he was well-known around Croxteth as "one of the boys". In 2006 he was given a three-month referral order for possession of a CS gas cannister and in February 2007 he was given an anti-social behaviour order for being part of a group terrorising security staff at Croxteth Sports Centre. There had been seven complaints about his behaviour in a matter of weeks.

Magistrates were told that Mercer and others, including his co-defendant Boy K, were engaging in a snowball fight in the centre in February last year and refused to leave the premises when asked. They threatened a security guard, attempted to set light to his jacket and told him they would get a knife to cut him.

Between 2004 and 2008 Mercer was stop-checked on the street routinely by officers on more than 80 separate occasions. In the month before he was charged with Rhys's murder, he was before a youth ourt for cannabis possession.

There is further evidence — unheard by the jury — of Mercer's willingness to use guns in pursuit of gang terror. Two months before the murder he rode into Norris Green, enemy territory, on a motorbike chasing rival gang members and waving a handgun in a "territorial and threatening fashion".

In court he cut an arrogant figure always ready to laugh and joke with his co-defendants when the jury was not there to see him. It was noted by Rhys's family, however.

Mercer's grievance against Wayne Brady, a senior Nogga Dog, was said to be personal — described in court as an intense hatred — but nothing to do with rivalry over a girlfriend. Brady had strayed on to Croxteth Park twice before and shots had rung out. His presence was a provocation.

Detectives in the case were anxious to gather as much material as they could to prove beyond doubt, and any possibility of a defence objection, that these teenagers formed the nucleus of a violent gang. By the end of their investigation they had gathered 3,000 pages of "bad character" evidence.

Mr Kelly said: "We are not dealing with the Mafia here. We are talking about kids, teenagers who were opposed to another gang who live on an adjoining estate.

"What is very disturbing is that it is anti-social behaviour but with very serious consequences. These anti-social people have access to firearms, and are not afraid to use them. The consequences are there for all to see".

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