The terrorist attacks on Brussels, which killed 31 people and injured another 270, have triggered calls for improved collaboration between Europe's intelligence services.
All three men behind Tuesday's bombings at the city's airport and a city centre subway station were known to police.
According to media reports, one of the three captured on CCTV cameras at Brussels Airport and who is still at large, was photographed on the Austria-Hungary border with the Paris attack suspect Salah Abdeslam on 9 September.
On Wednesday, EU commissioner Dmitris Avramopoulos, who is responsible for migration, home affairs and citizenship, said this week's horrors in the Belgian capital underlined the need for improved information gathering.
Speaking in Brussels, Avramopoulos said, "We cannot keep learning the hard way."
"It's a moment for all member states to start working together. To foster mutual trust, exchange information and intelligence, because this is the only way to go ahead."
In his first reaction to the atrocity, Avramopoulos added, "These events show, once again, that we absolutely need more coordination and more information exchange. The fact that the perpetrators of the Paris and Brussels attacks were known to the police proves this.
"Information gathering and information exchange are the cornerstone of our security. Our information needs to be interconnected. Our systems need to talk to each other. That is where we need to improve, urgently."
He went on, "What has happened in Brussels, has happened unfortunately already in Paris, in Madrid, in London, but also outside Europe in Turkey, Lebanon, Tunisia, Mali – and the list unfortunately goes on."
"But precisely because it has happened before, we know that each time the citizens of these cities, the citizens of Europe, have come out stronger and more united.
"To those that wish to divide us, we respond with more unity, cohesion and strength. A threat towards Europe needs a European response – and we have been calling for it for months."
A European Commission spokesman pointed out that the "European Agenda on Security", which sets an "effective EU response" to security threats, including tackling radicalisation, had been adopted some 12 months ago and that "significant progress" had been made in its implementation.
Even so, Avramopoulos' demand for better intelligence gathering was endorsed by senior British centre right MEP Tim Kirkhope who, while branding the two attacks "another example of the maniacs who threaten democracy and freedom in Europe" said they also reinforce the need for the intelligence agencies to work better together.
The ECR group deputy said, "I've spent the last five years taking legislation through this Parliament trying to improve co-operation between the intelligence services and the police in all member states, working together to detain and deter terrorists."
Kirkhope, a former UK government minister, added, "We have already achieved much, a great number of attacks have been prevented by this cooperation but the attacks are another chapter and show that we can never relax in our fight against terrorism.
"Balancing freedom with security is very difficult but as our sympathies go out to those who have lost their lives and the families of those who have been killed and injured, we must remain resolute, carry on our work and not be deterred by terrorism."
German Greens MEP Jan Philipp Albrecht agreed, saying there was an "urgent need for stronger police and security cooperation.
"Again," noted Albrecht, "the suspects were known by the police and obviously they were in contact to the recently imprisoned Abdeslam, suspect of the Paris attacks.
"Reflexively demanding for measures of mass surveillance does not prevent such attacks, those measures even block the view on the true deficits of security cooperation," said Albrecht, who is deputy chair of the Parliament's civil liberties committee.
"We need substantially more money and staff for across borders Joint Investigation Teams of Europol and Eurojust to get hold of the network of coordinated assassins. Furthermore, member state governments have to get willing to commit to common rules for the exchange of information and for handling with data of suspects and accused persons."
"Home affairs ministers have to send a clear signal to end voluntariness on the EU level."
In a rare united front, the leaders of Europe's main political parties joined forces in condemning the "barbaric and inhumane" attacks.
A statement issued by the four mainstream party groupings said such acts "cannot be justified by any cause or any ideology."
It read, "The recent attacks in Brussels and Paris, but also in Turkey, Tunisia, the Ivory Coast and others are sad evidence that we are living in a changing world, a world facing constant threats and challenges. We condemn ISIS and terrorist activities of all groups. The responsible for these attacks must be brought to justice. We must fight terrorism together because no country can handle this alone."
"What happened in Brussels is not only an attack on Belgium; it is an attack against Europe and against our shared founding values. It's an attempt to instigate fear, to alter our lifestyle and to challenge the peaceful coexistence in our society, to divide people and countries and to push each government and each state to fight on its own.
"We will not let this happen."
The statement goes on, "We, the leaders of the major European political parties, call for unity against terrorism and a joint action to ensure the safety of our citizens."
It is signed by five party leaders: Joseph Daul, of the EPP; Sergei Stanishev, of PES; Hans van Baalen, of ALDE and Reinhard Bütikofer and Monica Frassoni, co-chairs of the European Greens.
Further reaction came from British Socialist MEP Richard Corbett who was at Maelbeek metro station just a few minutes before the second bomb exploded and said, "There's a sense of dread here at Parliament as the attacks struck at places used daily by many friends and colleagues."
Elsewhere, a spokesman for the Dutch presidency of the EU said on Wednesday that it was "in close contact" with both the EU institutions and Belgian government to ascertain what "measures" need to be taken in the wake of this week's tragedy.
Dutch Prime Minister Mark Rutte said, "The threat level in our country remains substantial. There are no concrete indications that an attack is imminent in the Netherlands. Nevertheless in our country, too, we must be especially vigilant. Extra precautionary measures have been taken – some visible, others out of the public eye."
The attacks led to chaos for air travellers, with scores of flight cancelled or diverted from Brussels International airport, one of the busiest in Europe. On Wednesday, the airport, where the first attack happened, remained closed.
Geert Sciot, spokesman for the Brussels-based Association of European Airlines (AEA) said that "under very difficult circumstances" airlines were "doing everything they can to assist their passengers and meet their immediate needs."
He also sought to reassure passengers, saying, "Air transport is a key enabler of economic development, prosperity, connectivity and social cohesion. Tuesday's events had impact closer to home than ever before, hitting at the heart of Europe. It is essential that the air transport industry can thrive and function in a safe and secure environment. We trust that the air transport sector will demonstrate the necessary resilience to deal with this latest challenge."
Meanwhile, the European Parliament repeated its advice to staff to work from home if possible and said severe restrictions on entry will remain in place for the foreseeable future.
A spokesman for the institution said, "There will not be any exceptions to the current entry provisions."