In hour of need, can tech 'watchdogs' get job done?
FACIAL recognition software, scanners that detect weapons and cameras that spot nervous people are some of the technologies that could be used more widely to secure public places but some would require greater acceptance of surveillance in Europe.
The deadly attacks in Brussels highlighted the vulnerability of Europe's airports and transport systems.
European Union officials, grappling with the conundrum of how to increase security while retaining the openness of society, have convened meetings to discuss security.
Their goal is to be able to monitor passengers unobtrusively while minimising additional hold-ups.
Experts say technology cannot solve the problem on its own but techniques such as facial recognition able to pick out known suspects can help.
Technology security expert and academic Pierluigi Paganini said if properly applied, facial recognition technology could have flagged the bombers at Brussels airport.
Paul Murphy from IndigoVision, a British firm that specialises in video security systems, said a typical system could require 2,000 cameras and powerful computer servers.
"Only in the last two years has it become affordable and reliable," he added.
Such systems have been installed at Israel's Ben Gurion International, major hubs in the Middle East and an airport in South America, he said.
But the technology is still not as good as a person, he noted, and it could be hampered by simple measures like donning headwear. At least one of the bombers in Brussels was wearing a hat.
Recognising a suspect in real time is far harder than identifying a suspect after an incident.
"Comparing all of those faces (in a crowd) against a database is an enormously difficult task," said Kevin Riordan, UK director of checkpoint solutions at British airport scanner maker Smiths.
"Looking for a particular face in a crowd is easier."
Real-time identification requires the suspect to be known to the authorities and his information to be present in the database used by airports.
Elke Oberg of German software group Cognitec said the availability of data to be matched with images taken by cameras is a problem.
"For this, it's obvious security agencies should share more information, in compatible formats, which is rarely the case at the moment," she added.
The latest airport body scanners detect hidden objects with extremely high frequency radiation known as millimetre waves, which bounce off things they strike.
Britain's Qinetiq, a privatised former government defence research agency, is working on Passive Millimetre Wave technology that monitors naturally occurring energy from passengers, rather than requiring them to queue up and pass through a scanner that shines a beam at them.
It can find concealed weapons at a distance of up to 15m, the company says.
But any such technologies would still rely on security people on the ground able to respond in seconds.
"There are some technologies that are moving us forward but many of them rely on people making decisions," said Chris Phillips, former head of Britain's National Counter-Terrorism Security office.
Israel's Suspect Detection Systems is developing thermal cameras to flag suspects in a crowd by spotting unusual body heat, signalling anxiety.
Chief executive Shabtai Shoval, however, said surveillance must be combined with agents on the ground.
"For example, if set up at the entrance to a terminal to mark out potential terrorists, it would be of limited use if there are not security personnel available on the spot to intercept them," he noted.
One change on the table at the EU talks will be screening passengers at the entrance to airports, a process already in place in locations like Istanbul.
Mr Riordan said technology was not good enough to scan passengers as they entered a terminal without interrupting the flow.
"Millimetre wave technology can do that to a certain range but whether it can detect what is in somebody's rucksack is another matter."
Security portals, which passengers could walk through largely unimpeded, had been deployed, he noted, but their effectiveness was limited by the explosives used in attacks not being "very smelly".
A better solution might be more use of dogs.
But any application of technology would require a mind-set shift among civilians.
"In Israel, civilians feel like they are also on the front line," he said. "The question is whether the same can be said for Europeans."