"If the man shot in Milan is the Berlin killer, then the Schengen Area is proven to be a risk to public safety," Nigel Farage, the firebrand anti-European British politician, tweeted on Friday. "It must go."

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Not long before Farage's message, Marine Le Pen, the leader of France's far right National Front, published a blog post that dubbed the Schengen area a "security disaster," adding that France had been "reduced to learning after the fact that an armed and dangerous jihadist was probably wandering on its soil."

Meanwhile, Beppe Grillo, leader of Italy's euroskeptic Five Star Movement party, also wrote his own blog post that praised the bravery of the policemen in Milan before adding that Italy had become a "pathway for terrorists" thanks to Schengen.

The Schengen area is a grouping of 26 European nations that signed an agreement to abolish border controls between them and strengthen Europe's external borders. The 1995 agreement was originally separate to the European Union, but it has since become a part of E.U. law.

All but six E.U. nations have signed up, with the exception of Britain and Ireland, which opted out, and four other newer E.U. member states that are legally obligated to join. A number of non-E.U. states, such as Switzerland and Iceland, are also members. For many Europeans, the Schengen Area may be the most obvious example of European integration in their daily lives. In total, 400 million people can travel freely in an area that spans over 1.6 million square miles.

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Given Europe's history of division, borderless travel has a symbolic component, too. With no border controls, it can start to feel like Europe is one large and diverse country rather than a continent made up of nation-states. Writing in French newspaper Le Figaro last year, Jean-Claude Juncker, the president of the European Commission, said the Schengen area was "one of our very greatest achievements."

Yet even Schengen's supporters admit it can create problems. The open borders create opportunities for drugs and arms smuggling, making it easy and cheap to move illicit goods from one end of Europe to the other. “Ten years ago you wouldn't have found Kalashnikovs in Western Europe,” Nils Duquet, senior weapons researcher at the Flemish Peace Institute, recently told WorldViews.

The recent refugee crisis has placed even more strain on the system. A number of wealthy Western European countries have threatened to close their borders in response to waves of new arrivals from the Middle East and Africa. Poorer countries at Europe's periphery have struggled to cope with the number of people passing through in the hopes of claiming asylum in Western Europe.

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Amri was himself a newcomer to Europe, having left Tunisia after the country's 2011 revolution and taken an illegal boat across the Mediterranean to Italy. He quickly ended up in the Italian prison system, where he was moved multiple times because of problems with other inmates, the Associated Press reports. His family said that he tried and failed to find work in Switzerland after his release .

He then headed to Germany last year and applied for asylum. Amri's application was rejected, and he was the subject of a terror probe for his links to known extremists. However, moves to deport him were delayed because of his lack of a passport. Some reports say he had used false identities — at least six different names and three different nationalities — in his travels around Europe.

Amri was sought by German police after they discovered his identity documents in a wallet in the truck that drove through Berlin's Breitscheidplatz Christmas market. After he was killed, Italian authorities said on Friday that Amri had a train ticket to Milan that had left Chambery, France, the day before. It remains unclear why he had traveled via France.

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Despite the criticism, the Schengen area remains important for the day-to-day lives of many people in Europe. One of them was the Polish driver who was found dead in the truck used by Amri to attack the Christmas market. The unnamed driver had been due to deliver steel to Berlin. Investigators are assuming he was taken hostage by the perpetrator of the attack — and that, given signs of a struggle and the truck's erratic path, he may have died while trying to stop it.