With thousands of Europeans fighting in Syria and Iraq, governments have slapped unprecedented and controversial controls on would-be jihadists, but experts say a legacy of mistrust has hampered intelligence sharing.
Since the Islamic State group called on Muslims to come to the caliphate it declared a year ago, foreign fighter numbers have jumped, with the UN reporting a 71 percent spike in the nine months to April.
The International Centre for the Study of Radicalisation in London said the number of foreigners fighting in Syria and Iraq had already topped 20,000 by January -- with nearly a fifth of them from western Europe.
Governments have scrambled for a response, leaving many concerned about the balance between security and civil liberties, particularly when it comes to blocking people from travelling.
France has confiscated 60 passports since February and its lawmakers have approved sweeping new spying powers, allowing cameras to be placed in homes and devices installed on computers for anyone linked to a "terrorist" inquiry -- without a judge's authorisation.
Britain passed similar legislation in February, with a particularly controversial clause banning "extremist" preachers from talking at universities.
Germany, The Netherlands and Denmark have also begun confiscating ID cards and passports to stop suspected jihadists leaving the country, with Belgium expected to start soon.
- 'Need to display resilience' -
In balancing rights and security, many such as analyst Anthony Dworkin of the European Council on Foreign Relations fear the pendulum has swung too far away from civil liberties.
"It's important that our response to terrorism displays some resilience since we will never entirely get rid of the problem of terrorism," said Dworkin.
"A lot of these surveillance measures are not proven to make a productive difference. For instance, we know one of the main centres of radicalisation is the prison system, so locking up anyone who comes back from Syria is particularly counterproductive," he added.
A top counter-terrorism official at the Organisation for Security and Cooperation in Europe agreed that tough preventive measures could not be foolproof.
"It's relatively easy to travel abroad without a passport in the context of terrorists and returning foreign fighters using smuggling networks," said the official, who was not authorised to give his name.
The challenge, he said, was to keep operations "proportionate and non-discriminatory" and remain aware that "not all people with extreme views are necessarily terrorists".
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"Freedom of expression is a tricky thing. The threshold to criminalise speech should be very high."
While many are concerned about the erosion of rights, at least one parent feels it has not gone far enough.
A French mother this month took the government to court for failing to stop her teenage son joining jihadists in Syria, arguing that airport security should have stopped a 16-year-old with no baggage and a one-way ticket to Turkey -- the main entry point into the conflict.
- 'Terrorists can find loopholes' -
The big fear of governments is that these foreign fighters return battle-hardened and trained for attacks on European soil.
In a recent report, Britain's MI5 warned that more Britons than ever had been trained as "terrorists", and more than half the 700 who travelled to Syria have returned.
"The threat posed on their return comprises not just attack planning but radicalisation of associates, facilitation and fundraising," MI5 said in a report into spying powers.
Most agree the challenge is not so much collecting information, but being able to process it all and coordinate with other countries.
"The systems to share information exist, but they are not always used," said Camino Mortera-Martinez of the Centre for European Reform think tank.
"There are very different parts of Europe and they don't necessarily trust each other. MI5 may not want to give information to Romania, for instance, because they are worried about corruption and information leaking out."
A major sticking point has been sharing airline passenger information, which has been stalled since 2011 over concerns about data privacy.
Although 15 of 28 EU members have adopted their own data sharing systems, modelled on existing deals with the United States, Canada and Australia, the European Parliament insists that data protection laws must be adopted before the whole continent follows suit.
"If terrorists are a bit clever, they can find loopholes," said Mortera-Martinez.
"Given that agencies within the same country have trouble sharing information with each other, you can imagine how reluctant they are to share information with Europol or some other international body," she added.
"Intelligence services are not naturally very open people."