An MI5 spy used privileged status to terrorize his partner and then moved abroad, where he continued his intelligence work while under investigation, the BBC has revealed.
In the video, a man threatens to kill a woman and attacks her with a machete.
The foreign national cannot be named, despite strong evidence that he poses a threat to women, after the British government took the BBC to court to stop the story being published.
Evidence points to him being a right-wing extremist with a violent past.
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In an unprecedented court case, the BBC argued that the women had a right to know his identity and that this would protect potential victims from harm.
The BBC eventually managed to fend off the government's attempt to prevent publication of our extensive investigation.
Beth, a British citizen, met the agent through an online dating site.
The two began living together in Great Britain.
At first, she says, he was "charming".
They seemed to have a lot in common.
But over time it turned out that he is a misogynist and extremist, obsessed with violence and cruelty.
Beth, not her real name, says he sexually assaulted her and abused her and forced her to do things she didn't want to do.
She claims he used his privileged position in the British Secret Service to terrorize her.
“He had complete control over me. Today I am a pale shadow of what I once was," she says.
As their relationship became increasingly violent, Beth's mental health deteriorated.
"At the end of the relationship, he dictated what I should do every waking moment - where to go, who to see, how to work, what to do at work, what to wear."
X made her "feel absolutely worthless" and used "the fact that I had mental health issues to bully me and make me feel even more vulnerable."
He collected weapons and made her watch terrorist videos of executions and murders, she says.
"He terrorized me so much psychologically that it ended with me having a nervous breakdown, because I was afraid of everything - because of the way he made me think, because of the people he hung out with and because of the people he worked for."
She says X told her he was working as a paid informant for the British Secret Service - an agent or covert human intelligence source, according to the official terminology - infiltrating extremist networks.
She knew he was going to meetings with MI5 contacts, getting money and equipment from them.
At first, she said, he hid his real name from her.
Watch the video:
Our investigation established that X had spent years and years working as a source for MI5, using several false names.
The intelligence service manages agents in terrorist networks, informants who secretly cooperate with their contacts, members of MI5.
The controversial power gives agents the authority to commit crimes under their cover, but only to obtain intelligence that can save lives, prevent a more serious crime, or keep the agent safe.
But this does not apply to actions in their private lives.
Beth says he told her she wouldn't be able to report his behavior because of his status.
"That meant I couldn't talk about his behavior towards me, any violence I suffered, sexual or physical, because he had people in high positions who always had his back, who would intervene and who would practically kill me. killed, if I were to speak publicly."
X was paid to inform on right-wing activist networks, but the evidence we have seen suggests that his personal views were worryingly authentic.
Beth says that X praised various white supremacist mass murderers and talked about how he intended to commit similar acts himself.
At home, his violence was directed at Beth.
One video shows an MI5 agent attacking her with a machete.
Beth recorded the incident on her cell phone.
Just before he attacks her, she can be heard expressing her concern that she will be killed and saying that his treatment of her is unacceptable.
X says he's going to kill her, leaves the room, then returns with a machete in hand and raises it high above his head.
After she is heard telling him that the video will be handed over to the police, X goes on the offensive with guns and fists.
The video cuts out in the middle of her screams.
She manages to defend herself, but he attacks her again with a knife a few hours later, trying to cut her throat.
She says she bit his arm to stop him.
The police visited their home after the attack, but our investigation showed that there were serious failings in the police response.
X was arrested, charged with assaulting Beth, and brought before a judge.
However, while he was in court, the Crown Prosecution Service dropped the indictment.
Beth says that X returned to the apartment and continued to abuse her.
The police did not take a full statement from Beth or a video of the attack on her.
The Crown Prosecution Service quickly dropped the prosecution.
The police officers who participated in the case and the Crown Prosecution Service claim that they dropped the case due to a lack of evidence.
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After returning home, Beth says that X claimed that the Intelligence Service would stand behind him and that she was a "nobody and nothing".
Another recording, secretly made by Beth, shows a conversation with X in which he says that she could be killed for asking too many questions.
In the video, Beth asks if he's going to be the one to kill her.
His response suggests that he almost did during a previous violent episode.
Lounging haughtily on the sofa, he adds: “It's always in me. That killer instinct is always in me... I always imagine beating someone to death... It's always been in me... I've been doing it since I was six... I'm overly aggressive, and I'm aware of it myself."
Watch the video:
The two didn't stay together for long - X kicked Beth out of their apartment and disappeared while another investigation into him was still ongoing.
During a search of the home after the machete attack, local police officers found extremist material, including X's personal Nazi insignia.
The BBC has seen a police report detailing evidence found by officers, such as a private diary in which X wrote about killing "Jews".
He also wrote about how he was going to kill Beth.
Local police officers called in counter-terrorism detectives, who came and seized various items.
A terrorism investigation into X followed, but he left the UK while that investigation was still ongoing.
After only a few weeks of his departure, Beth had a nervous breakdown and ended up in the hospital.
While there, her belongings, which were also seized by counter-terrorism detectives, were returned to her family.
They were returned by a man who did not identify himself.
Her cousin assumed the man was an associate of X.
Neither Beth nor her family were aware that a terrorism investigation had been launched.
The family was disturbed by this visit - worried about how the stranger had found out about their address and how he had gotten hold of Betty's belongings.
We established that the visitor was an MI5 agent.
Material seized in the police investigation, under an official search warrant, was then passed on to MI5.
This extremely unusual move is consistent with the involvement of the intelligence service in criminal proceedings.
X's personal belongings were also handed over by the police to MI5, we gather.
Counter-terrorism police claim that no criminality was found during their investigation, but apologized for the fact that Beta's belongings were not directly returned to her.
After being released from the hospital, Beth filed a complaint with the local police department, asking why her case was dismissed, why her personal belongings were taken and why they were returned by an unknown person.
It took the police a year to hear her, but since then she has claimed that there is nothing to do because everything was investigated for the first time.
But that's simply not true.
During the appeal process with the police, Beth was told that the police had no information that any personal items were seized, but the "notes" showed that some items were taken and returned to Ix.
The police claim that they did not seize any items as part of their own investigation and that they "cannot say when or why they were taken".
That's not true, because the police officers involved in the case were the ones who called the counter-terrorism detectives.
The police claim that the items were returned to X is also untrue - the material was passed to MI5.
We found out that after X went missing, he moved to a foreign country and started working for a foreign intelligence service.
But X also has a history of shocking violence in the country - his extremist and violent behavior was known long before he became an MI5 agent.
We located and spoke with his ex-partner in that foreign country.
Her account of X's behavior is just as troubling as Beth's.
The two women never met or spoke to each other.
Ruth - not her real name - claims that X seemed normal at first, but eventually began to abuse and terrorize her.
He threatened to kill her and her child.
"He said he could kill both me and my daughter, and hide our bodies somewhere so no one would ever know who I was."
Fearing for her own safety, she sought help from a medical organization and was admitted to a safe house.
Due to the trauma, she could not speak at that time, so she was admitted to the hospital.
"I was psychologically broken, really broken," she says.
"I kept wondering why he had changed so much, but he once told me that this was his true face and that he had been pretending all along."
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We found a doctor who helped with Ruta's treatment.
He confirmed her testimony and described the events as perhaps the most drastic he had ever encountered.
He remembered the notebook in which X wrote down his desire to kill.
Ruth independently described the same object, saying that it contained fantasies of “blood, bones, flesh, human flesh. Eating children's meat."
He also threatened to kill and sexually abuse female children Ruth knew.
Later, Bet made the same threats.
The police in a foreign country have been informed about X and the threat he poses.
With a disturbing similarity to how he later abused his connection with MI5, X used the reputation of the criminal organization he was associated with to intimidate Ruth, telling her that its members could easily kill her.
X eventually disappeared, leaving her with only financial debts.
We discovered that he subsequently used her name for years in the UK, as a way of concealing his own identity, while on the permanent payroll of MI5.
"I don't think it's fair and I'm furious," says Ruth.
"I am very hurt. I feel cheated. You can't trust anyone. You can't get justice."
During our investigation, we found evidence independent of both women that corroborated their descriptions of X.
We found records in which X bragged about seriously sexually abusing women, using language too offensive to quote.
In other records, he approved the rape and murder of a young girl.
He openly registered on the website, looking for British women, under the identity he used to work for MI5.
Corroborating this, Beth told us how he described to her that he searched for women online to exploit.
Given that the BBC was able to establish that X had a history of violence and abuse, MI5 should have been similarly aware of this.
Last year, a serving police officer grossly abused his position to abuse, rape and kill Sarah Everard.
The whole public was concerned.
Government ministers also expressed concern - how such a tragedy could happen, just like the state's reaction to violence against women and girls.
An inquest into her death has been announced, with the Home Secretary saying it will "shed light on the failings that allowed a serving police officer to abuse his power in such an appalling way, as well as propose changes within the police force."
The warning signs given by X are convincing.
Both women we interviewed believe he is capable of killing.
But when we presented our findings to the government, they took us to the High Court, trying to stop us from publishing this story.
The court legally prevented us from naming Iks, due to the obvious risk to him from other extremists.
The government said it "will not comment on security or intelligence".
She added that the court order "is aimed at protecting national security and avoiding a real and immediate threat to life, security and privacy".
However, the BBC wanted to identify Ix so that women he was already in contact with or those who might meet him in the future could be warned.
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