Extra security checks are expected to delay hundreds of thousands of travellers this Easter weekend at airports and railway stations across Europe, following the Brussels terrorist attacks.

Many commentators have pointed to failures of intelligence agencies and a lack of screening at entrances to airports and underground stations as the factors that enabled terrorists to kill 31 people and injure some 300 in Brussels.

Yet experts do not expect the national and EU-wide reviews that have followed the attacks to yield major changes in security systems, as European nations continue their long-term battle to prevent periodic attacks inspired by the Islamic State extremist militia. 

"Across Europe we've seen a ramping up of security for a while," says Raffaello Pantucci, Director of International Security Studies at the Royal United Services Institute, a London-based defence and security think tank.

"This is not a new threat. People have known for some time," Pantucci tells dpa, citing the example of Britain keeping its threat level at "severe" - the second-highest level - for nearly two years.

Decisions over deployment of armed police on the streets, more bag searches and the use of airport-style security at other locations have already been taken, he says, adding that there could be "a bit more tinkering at the sidelines now in the wake of the Brussels attacks."

"Fundamentally, a lot has already been done and I think you're just going to see that continue," Pantucci says.

Jonathan Stevenson, a senior fellow at the International Institute for Strategic Studies (IISS), a global security think-tank, takes a similar view and expects "business as usual after Brussels." 

"There will be calls for international solidarity, tightened airport and ground-transportation security, less porous borders within and outside the EU," Stevenson has written in an IISS commentary. 

"Unless ISIS [Islamic State] follows quickly with another well-organized, highly lethal terrorist operation in Europe or the US, the trans-Atlantic security community will probably settle into the calm, if grim, dispensation that Brussels has confirmed the post-Paris threat assessment and reflects business as usual: periodic terrorist attacks on European soil," Stevenson says.

Speaking in Brussels on Wednesday, EU Migration and Home Affairs Commissioner Dimitris Avramopoulos said a lack of shared intelligence meant the terrorists who attacked Paris in November could not be stopped, even though "they were all somehow known to the local intelligence authorities."

In addition to possible gaps in intelligence sharing between nations, Pantucci says vital information "doesn't always filter down to the right people" on the front-line of policing.

"These sorts of attacks are difficult to deal with for many reasons, the most obvious one being that you're dealing with very secretive organizations, very determined people who want to try to kill us and who want to die while they do it," he says. "So that makes it quite a challenging opponent to have to face."

Bernard Hogan-Howe, the head of London's Metropolitan Police, told The Times following the Brussels attacks that good intelligence was crucial because the "terrorists have proved to be creative." 

Britain's national lead officer for counter-terrorism, Mark Rowley, warned earlier this month that Britain faces an ongoing threat of "enormous and spectacular attacks" by Islamist terrorists.

In Britain and other EU nations, the Brussels attacks have revived debates over control of firearms and spurred politicians to expedite a scheme to share European air passenger name records.

The severity of the attack on the Bataclan theatre in Paris in November, in which 89 people died, was "really a product of those guys [terrorists] having machine-guns," Pantucci says, highlighting "relatively easy access to weaponry" in continental Europe.

Measures such as screening at airport entrances were discussed following the July 2005 "7/7" terrorist attacks on public transport in London, but the government and aviation businesses argue that this would only move the terrorists' targets to different locations at the transport hubs.

"I don't know that anything's changed that would make people reassess that," Pantucci says. "If there was perhaps a regular bombing campaign then, yes, it might be discussed [again]." 

Stevenson says the Brussels attacks were "a tragic reminder that Islamist terrorists will not go quietly or quickly," despite tougher policing inside Western nations and nearly 10,000 airstrikes by a US-led coalition on Islamic State forces in Iraq and Syria. 

He warns that "Islamist terrorist infrastructure in Europe long predated ISIS and is not easily dismantled." 

"The reality, however frustrating, is that, under any of several plausible hypotheses about ISIS's motivations, it is difficult for Washington and European capitals to do much more," Stevenson says. 

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