A care worker and amateur tattooist has been found guilty of murdering a 13-year-old girl after she threatened to reveal that he had been sexually abusing her for more than a year.

Stephen Nicholson, 25, a lodger at the family home, lured Lucy McHugh into woods in Hampshire and stabbed her 27 times before hiding the weapon and trying to burn the clothes he had been wearing.

Detectives believe Nicholson carefully plotted the murder, characterised in court as an “execution”, to silence Lucy, and meticulously planned his attempt to get away with it.

After Lucy vanished, Nicholson sent her mother an apparently supportive message in which he told her to keep her chin up and coolly got on with a DIY project to convert a shed into a reptile house for his pet snakes, which included a 2.7-metre (9ft) boa constrictor.

A serious case review is under way to examine what contacts agencies had with Lucy after it emerged that teachers had flagged up to social workers concerns that she was spending time unsupervised with older men.

The case also focused attention on how difficult it is to obtain details of Facebook messages even following the most serious of crimes. It was only as the trial started, almost a year after Lucy’s death, that confirmation finally came through that she and Nicholson had communicated via Facebook shortly before she was killed.

Lucy McHugh. Photograph: Hampshire Constabulary/PA

Lucy was a bright and bubbly girl. At her funeral, her family portrayed her as a “little hurricane of smiles and lip gloss, wearing the brightest colours possible, with no care for what anyone thought”.

In 2017, Nicholson became a lodger at the Southampton home of Lucy, her mother, Stacey White, and stepfather, Richard Elmes, who was Nicholson’s best friend.

The prosecution at Winchester crown court said that by this time he had already sexually assaulted a 14-year-old girl in 2012, when he was an 18-year-old door-to-door salesman, after she asked him for a tattoo.

Diary entries from Lucy reveal that he began abusing her in May 2017, when she was 12, after they had played the video game Call of Duty: Zombies.

In July 2018, Nicholson lured her to woods near her home in Southampton and killed her. A dog walker found the body.

Nicholson insisted he had nothing to do with the crime but police found letters including one from Lucy to Nicholson found the day after she died that read: “To Stephen … I think I am pregnant.” Postmortem tests revealed she was not pregnant.

Another note was found that had gone through the family washing machine with the title: “Abuse”. Much of it was illegible but one part that survived read: “Walk away from my sex with him, forced me back in.”

Police came to believe Lucy was killed after threatening to reveal she had been abused by Nicholson.

Detectives were keen to find out what the pair had been saying to each other on Facebook but Nicholson refused to give police his password and he was jailed under the Regulation of Investigatory Powers Act. Detectives were forced to go to Facebook and the US Department of Justice to try to get access to the messages.

Meanwhile, a partly-burned hoodie and tracksuit trousers that Nicholson had changed into for the killing – which detectives called his “murder kit” – were found in a stream a 15-minute walk from where Lucy had been killed. DNA from Lucy and Nicholson were found on the clothing.

Police carried out what is believed to be the largest ever fingertip search – 600,000 sq metres of land – and examined thousands of hours of CCTV footage. They also traced Nicholson’s movements using data from his phone. But the closest police have got to finding a murder weapon was discovering two pieces of plastic from a knife handle.

The case raises questions over how concerns about Lucy were dealt with. Her school regarded her as vulnerable and contacted social workers because of fears that she was spending time alone with men at home. Social workers spoke to Lucy’s mother but the jury heard she told them there was nothing to worry about.

Speaking outside court, DS Paul Barton, of Hampshire police, would not comment on the contacts between Lucy and other agencies. “We need to wait for the results of the serious case review,” he said.

Barton described Lucy as a clever and promising girl. “She was popular and doing well at school with lots of friends,” he said.

But he believed Lucy had become infatuated with the man who was abusing her. “He used and abused her. He had no further need for her. She had feelings for him. I don’t think he had any feeling for her or anyone else. I think she gave him an ultimatum: if you’re not going to be with me I will tell people. The only way to silence her was to kill her.

“The most chilling aspect is the planning. He gave some thought to the what-ifs. She probably went to meet him with a spring in her step thinking she was going to rekindle some kind of relationship.”

John Montague, senior district crown prosecutor, called for the process of getting information from companies such as Facebook to be speeded up in such serious cases. “It’s a protracted process,” he added.

A spokesperson from Southampton city council said: “The tragic death of Lucy McHugh will be subject to a serious case review.”

Nicholson was convicted of murder and three counts of rape when Lucy was aged 12. He was also found guilty of sexual activity with a child in relation to a 14-year-old girl in 2012.

Barton added: “I would describe him as cold and calculated. I would describe him as a paedophile and I think he is someone who only thinks about himself and has taken full advantage of this family that have looked after him, provided a roof over his head.

“He has targeted Lucy, taken advantage of her, and when she wanted a relationship with him, he has taken the decision to silence her once and for all by brutally killing her.”

On the Facebook issue, the Metropolitan police commissioner, Cressida Dick, said officers were forced to go through a “very protracted procedure”, and added: “I absolutely think that in certain instances, and it sounds to me like this is one, law enforcement in the UK ought to be able to have vital evidence which might bring somebody to justice.”

A Facebook spokeswoman said: “We have worked closely with Hampshire police throughout this case and have provided information they requested through the mutual legal assistance treaties.

“This is one of the legal mechanisms for the police to obtain information in certain circumstances. We agree that this legal process can be far too slow which is why we have actively lobbied for reforms to EU, US and UK laws to allow us, and others, to directly and more quickly provide information to UK law enforcement authorities.”

The judge said Nicholson would be sentenced on Friday.