LONDON — The journey of British Parliament attacker Khalid Masood from criminal to deadly terrorist underscores the challenges faced by security services in identifying and tracking extremists.
The 52-year-old, who killed an American tourist, a police officer and two others in a car-and-knife rampage outside the House of Commons, had previously been investigated by intelligence agencies in the U.K.
His emerging story is chillingly familiar, coming after a string of atrocities in London and elsewhere in Europe committed by suspects known to the justice system but who remained free to wage jihad on the streets.
Police chiefs on Friday appealed for help from the public in finding out how the attacker — born Adrian Russell Ajao in the suburban English county of Kent — became radicalized.
In the hunt for clues, police made two "significant arrests" overnight, bringing the total in custody since Wednesday's attack to nine.
"Clearly a main line in our investigation is what led him to be radicalized," Metropolitan Police Acting Deputy Commissioner Mark Rowley told reporters. "Was it through influences in our community, influences from overseas or from online propaganda? Our investigation and arrests will help in that but the public appeal will make a real difference."
Masood plowed a rented 4x4 into pedestrians on London's Westminster Bridge and fatally stabbed a policeman inside the gates of Parliament before being shot to death by an armed officer.
Masood had convictions for offences including assault and possession of offensive weapons and was investigated "some years ago" by Britain's domestic spy agency MI5 but was "not part of the current intelligence picture," according to Prime Minister Theresa May.
Maj.-Gen. Chip Chapman, a counter-terrorism expert and former senior British military adviser to U.S. Central Command, suggested the attacker may have been overlooked due to this age.
"His profile is highly unusual," Chapman told NBC News. "The average age for offenders of this kind between 1998 and 2015 is 22. Perhaps there will be an adjustment to the way extremists are profiled."
He added: "There is a prioritization grid and for some reason he wasn't near the top, which means there are also a lot of other malevolent people who are higher up the list."
Masood, who was never convicted of any terror-related offenses and wasn't under surveillance, reportedly served at least one jail term.
"One theory is that Masood may have become radicalized in prison or from a contact made in prison," Chapman said, noting that more than 1,000 known extremists were currently in Britain's jails.
A long list of terrorists have been traced back to jails, including the two perpetrators of the Charlie Hebdo massacre in Paris who fell under the spell of a convicted jihadi described as a "sorcerer" inside Europe's largest prison. Experts have warned penitentiary facilities could become "universities of radicalization."
Britain's Home Secretary Amber Rudd said it would be wrong to say there had been an intelligence failure over Masood, adding that security services had foiled 13 terror attacks since 2013.
Lawmakers are reportedly investigating if Masood should have been tracked, although such monitoring failed to stop the man who beheaded off-duty soldier Lee Rigby in 2013 and the 2005 "7/7" London transit bombers.
Experts say the sheer number of known extremists makes constant surveillance impossible — as well as politically undesirable.
"Actually watching somebody 24 hours a day, seven-days-a-week is incredibly intensive and difficult," former Home Secretary Jack Straw told ITV News. "There are some who think, trailing people, you need a couple of guys ... but it's not like that at all. It's really complicated to be able to follow somebody."
Bernard Hogan-Howe, the former commissioner of London's Metropolitan Police, said that the sheer number of individuals being monitored means cases have to be prioritized.
"No doubt, it's hard. It's a really difficult decision," he told NBC News last year. "We have got many people who we are worried about ... either involved in or supporting terrorism so all the time we are trying to assess: 'Are they thinking about it? Are they getting together equipment to do something about it? Are they working with other people to conspire to organize it?'"
Similar questions were raised when college graduate Mohammed Emwazi fled London to join ISIS despite being on Britain's Home Office Warnings Index as a petty criminal who could potentially become radicalized.
The would-be soccer player was unmasked in 2015 as ISIS executioner "Jihadi John" who gained notoriety in a video showing him beheading U.S. journalist James Foley in Syria.
Most recently, French authorities had to explain how two known extremists, including a teen who twice tried to wage jihad in Syria, were released from prison on bail and went on to slit the throat of an 86-year-old priest in church.
French Prime Minister Manuel Valls explained that intelligence agencies were monitoring more than 600 French nationals fighting in Syria or Iraq, including 283 women and 18 children, and some 300 more who had returned to live in France. "Each day [we] ... trace networks, locate cells, arrest individuals," he said.
Governments across Europe have been tightening anti-terrorism laws and agreeing to share more intelligence in a wider fight against militancy both in the Middle East and a home.
With the threat level at "severe" — meaning an attack is "highly likely" — London's Mayor Sadiq Khan acknowledged last year that the risk of terrorism was "part and parcel" of life in a big city.
"It is a reality I'm afraid that London, New York, other major cities around the world have got to be prepared for these sorts of things," he said in comments that were ridiculed on Wednesday by Donald Trump Jr.
"Ultimately, in a free and democratic society, there is going to be a balance between democracy, freedom and openness and a police state," Brian Dillon, the former operational head of the Specialist Firearms Command for the Metropolitan Police, told NBC News. "We can't live in a police state and therefore at some point some attacks are regrettably going to hit home. That's inevitable. Not everything can be stopped."
He added: "There is a vulnerability in any city, in any urban environment where people are gathered."