Khalid Masood's criminal past is a common marker for recent jihadists, but the London attacker's age of 52 is unusual and there was little in his profile that could have given an early warning, experts said.
"There aren't any surprises," said Brooke Rogers, a terrorism researcher at the International Centre for the Study of Radicalisation (ICSR) at King's College London.
"The link to past criminality, we have seen that before," she told AFP, adding that if his reported stints in prison were confirmed, they could have been "a potential entry point for radicalisation".
Masood's age is the only outlier, as younger jihadists are "more the norm", but even that has been changing in recent years, Rogers said.
But Jason Burke, a Guardian journalist who has written extensively about Islamic extremism, said Masood's age was a "standout detail" as it made him nearly twice as old as most contemporary attackers.
Masood ploughed through dozens of pedestrians on Westminster Bridge on Wednesday, then ran out of his car armed with a knife and stabbed a police officer guarding parliament before being shot and killed.
Four people died in the attack.
Prime Minister Theresa May on Thursday said Masood had been known to the domestic intelligence agency MI5 as a "peripheral" figure several years ago "in relation to concerns about violent extremism".
But she said there was no early warning of his threat.
The history of jihadists with non terror-related criminal backgrounds is well documented.
In a report last year, the ICSR said prisons in Europe were a "breeding ground" for jihadist groups.
Some criminals see violent extremism as a form of redemption for their crimes, according to the study.
ICSR director Peter Neumann said at the time that "a lot of networking" for jihadists happens in prison.
Researchers from the ICSR compiled profiles of 79 European jihadists with criminal pasts from Belgium, Britain, Denmark, France, Germany and The Netherlands.
Of those studied, 57 percent had been incarcerated before being radicalised and 27 percent of those who spent time in prison were radicalised behind bars.
On Friday, Neumann said that "except for his age, Masood is exactly the kind of person" in the report.
- 'Community awareness' -
Still, experts said there was little in his profile that could have marked him out as a threat ahead of time, after British media outlets reported that he was not on a list of 3,000 suspected extremists who were being regularly monitored by intelligence services.
James de Waal, a senior fellow in international security at the Chatham House think-tank, said the trouble was that the number of people radicalised in prison was "a very large group of people".
The answer, according to some experts, is greater monitoring of online activity and better communication with the public to encourage people to come forward about anyone with extremist views.
"We can step up security measures, we can have more armed police, but unless community engagement measures are as good, people like this man can fall through the cracks," said Lina Khatib, head of the Middle East and North Africa programme at Chatham House.
Britain's counter-terrorism strategy, known as CONTEST, includes the Prevent programme aimed at countering radicalisation.
As of 2015, there is now a legal obligation on doctors and teachers to report people showing signs of radicalisation.
But getting the public on board is more difficult.
Announcing investigation details on Friday, Britain's top counter-terrorism officer, Mark Rowley, made a strong appeal for anyone with information.
"There might well be people out there who did have concerns about Masood but weren't sure or didn't feel comfortable for whatever reason in passing information to us," he told reporters.
Rogers, at King's College, said that Britain did not have "thought police" and that the emphasis should be on more "upstream" intervention and community engagement.
"This is a lot about community awareness," she said.