The Brianna Ghey case should prompt parents to “sharpen up” their internet awareness and police what their children are viewing online, the detective who oversaw the investigation into her teenage killers has said.

Girl X – the 15-year-old who befriended Brianna and then plotted to kill her – had downloaded a special browser on her phone in order to access the dark web and watch snuff movies, the trial heard.

She and her accomplice, Boy Y, traded thousands of messages about torture and killing, a jury at Manchester crown court heard. They discussed murdering various boys they did not like, before settling on 16-year-old Brianna, a transgender girl, as their victim – largely, police believe, because she was “accessible” and “vulnerable”.

Det Ch Supt Mike Evans, head of crime at Cheshire police, said reading through their messages made him wonder: “How do you know what your kids are doing when they are sat with the door shut in their bedrooms?”

He added: “I can only reflect on myself as a parent, thinking, actually, how internet aware am I? And do I know what my kids are looking at – inadvertently or advertently? I think we could all use this as an opportunity to sharpen up.”

Brianna’s murder should make everyone more vigilant about what children are viewing online, he said. “But I don’t for one minute think we’ve got kids across Merseyside and Cheshire sat there Googling how to kill people. We’ve got to keep this in the context of what it is, which is horrendous. It’s two warped individuals.”

Giving evidence in court, Girl X admitted that she liked watching videos of “dark materials”. She told the jury she accessed the dark web via the Tor Onion Router browser app, which she bought when she was 14, and used it to view “a mixture of violent stuff, including murder and torture”.

Two months before the killing, Girl X sent Boy Y a video, which was an advert for an underground site for people who like “rape, snuff, torture and murder”, and then told him: “I love watching torture vids. Real ones on the dark web.” She said she had found a good online “red room”, where people are tortured and murdered for an audience of viewers via the dark web.

X and Y’s web-savvy behaviour showed their intelligence, Evans said. “How many 14-, 15-year-olds can access the dark web? They’re not thick.”

The detective said he did not think anything could have been done to prevent the murder. “Did any warning signs get missed by anyone around those two individuals? I don’t think so. You look at the text messages. This was a life they lived, very closed between each other. They didn’t discuss it with other people.”

Evans said there was one potential missed opportunity concerning an incident a few weeks before the killing, when X told Y that she had poisoned Brianna with an overdose of red ibuprofen tablets.

The court heard that Brianna was “very sick” around that time and vomited up what her mother, Esther, thought at the time was red grapes but in hindsight may have been the tablet casings.

“Mum acted as you would expect mum to act: she supported her, looked after her. Why at that point would anyone think you’d been poisoned? It doesn’t make any sense. It’s only now when we’ve got the jigsaw that we’ve got that it looks suspicious,” said Evans. “I genuinely don’t think anyone could have done anything to stop [the murder], other than these two, who did what they did.”

Evans defended Cheshire police’s decision to say very early on that they did not believe Brianna’s murder was motivated by transphobia. “It would have been very easy to jump and say: ‘Yeah, this is a hate crime and therefore we need to start looking at people who target transgender kids.’”

He said that, had the force done that, “we wouldn’t be sat here today in a position where we are with these two now convicted. By following the evidence, it took us to them, and they have rightly been convicted.”