A police officer guards the entrance of the judicial police headquarters in Paris. The French prosecutor's office said Salah Abdeslam, the key suspect in the Paris attacks, was transferred from Belgium to France on Wednesday morning. Michel Euler/AP hide caption

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Belgian police officers stand in Albert I square in Brussels, where authorities arrested Mohamed Abrini, a key Paris attacks suspect, on Friday. Thierry Charlier /AFP/Getty Images hide caption

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The Belgian Federal Police released surveillance video of three men suspected of taking part in the attacks at Belgium's Zaventem Airport, including the "man in the hat." AP hide caption

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Mohamed Abrini (top right) is seen with Salah Abdeslam in this image taken from a CCTV camera at a gas station north of Paris on Nov. 11, 2015. Wanted for questioning over both the Paris attacks and the recent bombing of the Brussels airport, Abrini was arrested Friday. AFP/Getty Images hide caption

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Police detain a group of people at the Place de la Bourse in Brussels, Belgium, Saturday. Authorities had banned all marches in Brussels after a far-right group announced its plans to hold an anti-Muslim rally in the city. Geert Vanden Wijngaert/AP hide caption

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A masked man raises a fist and the Belgian flag outside the stock exchange in Brussels on Sunday, at a square filled with memorials to the victims of Tuesday's terrorist attacks. Tensions mounted after the square, occupied by mourners, was invaded by far-right protesters whom AFP identified as football hooligans. Patrik Stollarz/AFP/Getty Images hide caption

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The Belgian flag flies at half-mast at the Koningin Astrid-Reine Astrid military hospital in Brussels on March 24, 2016. Some of the people wounded in the Brussels attacks are being treated there. Patrik Stollarz/AFP/Getty Images hide caption

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People gather Friday at a makeshift memorial in tribute to the victims of the Brussels terror attacks, on Place de la Bourse square in Brussels. Grieving Belgians held prayers in the rain for the victims, but there was also growing anger at the government for letting a string of militants slip through the net. Laurie Dieffembacq/AFP/Getty Images hide caption

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Ibrahim el-Bakraoui (center) appears in this image provided by the Belgian Federal Police in Brussels, on March 22. Turkish authorities say they warned both Belgium and the Netherlands of his terrorist links when Turkey deported him last year. AP hide caption

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U.S. Secretary of State John Kerry stands alongside Belgian Prime Minister Charles Michel during a meeting in Brussels on Friday. Laurie Dieffembacq/AFP/Getty Images hide caption

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