UK security services are facing mounting questions over claims that the British graduate believed to be murderous extremist Jihadi John was known to MI5.
Reports claimed to unmask west Londoner Mohammed Emwazi as the Islamic State (IS) frontman seen posing in several barbaric videos of hostages being murdered.
He was described as "extremely kind" and "extremely gentle" by a former confidante, but family members of his victims, including US journalist Steven Sotloff and British aid worker David Haines, called for him to be brought to justice.
Asim Qureshi, research director of British advocacy group Cage, claimed Emwazi was interrogated by MI5 and subjected to security agency harassment before becoming the now-infamous militant.
Sir Menzies Campbell, a member of the Intelligence and Security Committee (ISC), said the parliamentary committee was likely to seek answers from the security services about what information they held on Jihadi John, but not until after the general election.
Reacting to reports, Mr Haines' wife Dragana told the BBC seeing the extremist captured would give the families "moral satisfaction".
She said: "Ever since we found out David had been murdered I was hoping that this man would be identified and be caught, but it's difficult to be reminded of it all again.
"I hope he will be caught alive - I think that's the only, I don't know, moral satisfaction of all the families of all the people that he murdered because if he gets killed in heavy action it will be an honourable death for him and that's the last thing I would want for someone like him.
"I think he needs to be put to justice but not in that way."
But Mr Haines' daughter Bethany told ITV victims' families would feel closure only "once there's a bullet between his (Emwazi's) eyes".
Meanwhile, a spokesman for the family of Mr Sotloff said: "We want to sit in a courtroom, watch him sentenced and see him sent to a supermax prison."
Earlier, Mr Qureshi said he had come to know "beautiful young man" Emwazi before he fled for Syria.
Mr Qureshi said the country's national security policy "only increased alienation" since the 9/11 terrorist attacks and cited similarities between the case of Emwazi and that of Michael Adebolajo, who murdered soldier Lee Rigby in Woolwich in 2013.
Speaking at a press conference, Mr Qureshi described Emwazi as "the most humble young person that I knew". He said: "He (Emwazi) was such a beautiful young man, really. It's hard to imagine the trajectory, but it is not a trajectory that's unfamiliar with us.
"We've seen Michael Adebolajo, once again somebody that I have met. He came to me for help, looking to change his situation.
"When are we going to finally learn that when we treat people as if they're outsiders, they are going to feel like outsiders and they will look for belonging elsewhere?"
He said Emwazi would turn up at Cage's offices with "posh baklava" as a way of saying thanks for assistance during his battles with security services.
And in a swipe at authorities, Mr Qureshi added: "A narrative of injustice has taken root. A narrative of impunity that there is no accountability for the way in which our security agencies operate. Unless we arrest that narrative, we are just going to see these things happening over and over again.
"People will feel like they are pushed out and that they don't have a place to belong. And when somebody is giving them a message, 'Come, we will give you a sense of belonging', how can argue against that?"
Sir Menzies told BBC Radio 4's The World At One: "I think it will be for the committee formed in the next parliament to ask for a report and then, if it thinks it necessary, to take evidence from the relevant security services."
Sir Menzies told Radio 4 that the ISC had not been informed of Jihadi John's true identity and would not expect to have been.
"I'm totally unaware of that and I wouldn't have expected it to be the case, because although the committee is entitled to some evidence about operational activities, that is by and large after these operations have been concluded," he said.
Sir Menzies said the case appeared to have "echoes" of the 2013 murder of Lee Rigby, as killer Adebolajo later turned out to be known to security agencies.
"One of the difficulties here is you can't keep an eye on everyone all the time, and as the committee found in the case of Lee Rigby, there's no doubt that from time to time the security services have got to prioritise those upon whom they are conducting surveillance," he said.
Jihadi John rose to notoriety after he first appeared in a video posted online last August, in which he appeared to kill the American journalist James Foley.
Dressed all in black with a balaclava covering all but his eyes and the ridge of his nose and a holster under his left arm, he reappeared in videos of the beheadings of US journalist Steven Sotloff, British aid workers David Haines and Alan Henning, and American aid worker Peter Kassig.
And last month, the militant appeared in a video with the Japanese hostages Haruna Yukawa and Kenji Goto, shortly before they were killed.
Kuwaiti-born Emwazi is believed to have travelled to Syria in 2013, according to Cage.
The organisation said his family was in "utter shock" and unable to accept that Emwazi could be Jihadi John.
It is believed he later joined IS, which has taken control of large swathes of the conflict-torn country, as well as territory in neighbouring Iraq.
Scotland Yard has refused to confirm reports of Emwazi's identity.
Commander Richard Walton, head of the Met's counter terrorism command, said: ''We have previously asked media outlets not to speculate about the details of our investigation on the basis that life is at risk.
''We are not going to confirm the identity of anyone at this stage or give an update on the progress of this live counter-terrorism investigation.''
Downing Street declined to confirm or deny that the reported name was known to the intelligence and security services.
Asked if David Cameron was concerned about Emwazi's name being reported, a No 10 spokeswoman said: ''The Prime Minister would be concerned about information being put into the public domain at any time that might jeopardise ongoing police or security investigations or the safety of British citizens."
Mr Qureshi said part of him "still hopes that we have just made an astronomical mistake" about Emwazi.
Asked if he condemned what Emwazi was doing, he told Channel 4 News: "Of course I condemn the actions of killing people and assassinating or executing them. That's not really the way in which I think that they should be going about doing things."
Pressed to specifically condemn Emwazi's actions, he said: "As far as we can see, I can't say that for certain because the video that I've seen, it cuts away. But if it's true..."
Two British trainee medics who say they met Emwazi in Syria said he was "generous and caring" with his friends but "always ready for war".
The men claimed they met Emwazi when he visited friends in a hospital they worked in months before his alleged role in IS execution videos emerged.
One told ITV News: " I remember he was quiet, not reserved quiet, he had a lot of friends and was social.
"In Syria he seemed to be a very busy man, he was always ready for war in safe areas.
"Most fighters are constantly armed but he always had full gear on as if he was going off to battle. This very instant. Which is quite rare in the safer areas in Syria.
"He was always on the phone busy, doing something. But always made time to visit his injured friends in hospital. "He was generous and caring with his friends."
He had no intention of returning to London and gave a "scowl" at the mention of the UK, one of the medics, who have not been named, said.
The man the two men encountered was said to have been unmarried and appeared to be "wealthy".
"All of his kit was expensive," said one. "Even the guns he had were extremely expensive and rare to find in that part of the world."
Mr Foley's mother Diane told The Times she forgave her son's killer.
"It saddens me, his (Emwazi's) continued hatred. He felt wronged, now we hate him - now that just prolongs the hatred. We need to end it," she said.
"As a mum I forgive him. You know, the whole thing is tragic - an ongoing tragedy."
The paper also quoted Tory MP Sir Gerald Howarth as saying security services should explain how the militant was "able to slip through the net".
He said: "If they are thought to be that valuable that we try to recruit them, if they are that important they need to be watched as well. There must be a system whereby they can be intercepted before they leave the country."