Some of the leaders of self-styled paedophile-hunting groups operating in Northern Ireland have been identified for the first time.
There are increasing concerns about the methods many of the groups use, and their lack of accountability.
The so-called paedophile-hunting groups target people online who they believe are sexual predators.
They confront them and broadcast the encounter live on the internet, then call the police.
There have been more than 100 incidents of that nature reported to police in Northern Ireland, but no-one has been charged as a result and there have been no convictions.
Former senior police officer and child protection expert Jim Gamble warned that some of the tactics used by the groups are criminal.
"Anyone live streaming these incidents is not about the justice of catching someone who represents a threat to children," he said.
"They are about the self-publicity and the self-centred approach about themselves, much more than about making children safer."
All of the groups keep their identities hidden.
'On whose authority?'
BBC News NI approached two of the people involved in setting up the "hunting groups" in Northern Ireland to ask them for an interview.
Sharon Shanks, from south Belfast, is behind the group called Justice Reborn Northern Ireland and she uses the alias Chelsea Lewis.
Ms Shanks declined to do an interview, saying in an online message that she was unhappy that "journalists and news stations have so far called the hunting groups vigilantes".
"Our identity we keep to ourselves so meeting isn't an option," she said.
During one confrontation that she posted live online, Ms Shanks is overheard using explicit offensive language and threats of violence against a man who came across the scene.
When approached by the BBC just after she had broadcast other live footage on Facebook of herself interrogating a man she had alleged was a paedophile, Ms Shanks refused to answer our questions.
We wanted to ask about the methods her secret group used, and on whose authority she had been detaining people who she believed to be paedophiles.
She declined to answer any questions.
Instead, Ms Shanks pulled on a scarf and was driven way at speed.
There are up to 10 different, self-appointed paedophile hunting groups operating in Northern Ireland.
Often they are in competition with one another, but sometimes they join forces.
'Won't answer questions'
One of those who introduced the concept of "paedophile hunting" to Northern Ireland is 34-year-old Belfast man George Keenan.
He uses the alias James SJ O'Neill and was linked to a group calling itself Silent Justice.
One of his targets last August was a man in County Antrim.
Two days after he was confronted, the man took his own life.
His family said they wanted to maintain a dignified silence.
Mr Keenan recently had a public falling out with other paedophile hunting groups in England and, in an expletive filled video, issued various threats of physical violence against them on Facebook.
Mr Keenan declined a request for an interview using his own name and identity.
BBC News NI approached him in west Belfast to ask him on whose authority he was confronting people he thought were paedophiles.
When asked if any of his actions resembled those of a vigilante, he replied: "Are you serious? Get out of my sight, leave me alone.
"Stay away from anybody that I have previously worked with.
"You are harassing people, I am not here to answer your questions.
"You are absolutely disgusting. Get out of my space right now."
Later the same day, Mr Keenan led a group of people to confront BBC investigations reporter Kevin Magee in a coffee shop in Belfast.
Mr Keenan claimed he had been able to mobilise more than a dozen people from various paedophile hunting groups.
For a short time they blocked off Botanic Avenue and published the episode online.
Eyewitness account: 'Terrible, outrageous behaviour'
A mother-of-two has told of her terror when her home was mistakenly surrounded by a group of masked paedophile hunters.
The self-appointed gang surrounded the house in the Springfarm estate in Antrim in January, claiming that a paedophile was living there.
But the group had made a mistake - they had picked the wrong house.
The 34-year-old woman, who asked not to be named, said she was "absolutely terrified" when members of the gang approached her home.
Her 66-year-old mother and two young children, aged seven and three, were in the house with her.
"A crowd of people got out of cars and they started banging and kicking at my front door - one of the women was screaming," she said.
"They were all saying there was a paedophile in the house, but there was no man in the house, just me, my kids and my mother.
"It was terrible, it really was outrageous behaviour - we felt really intimidated.
"One of the children was hiding under the bed and my mother was petrified and shaken afterwards.
"Now she doesn't want to leave the house."
The woman had only recently moved into the house.
'Job for police'
Neighbourhood watch co-ordinator Seamus Davis went to the house when he heard the commotion.
He said: "There were three or four cars full of people.
"I said: 'Look you're in the wrong place' - and they called me a paedophile lover.
"I told them I hate paedophiles as much as anybody else, but this way you are doing is totally wrong - this is a job for the police.
"You are behaving like a bunch of vigilantes."