23 photos: World reacts to Brussels bombings  23 photos: World reacts to Brussels bombings  Belgian national flags are projected onto the National Gallery in London's Trafalgar Square on March 23.  23 photos: World reacts to Brussels bombings  A woman in Brussels pauses after people observed a minute of silence at the Place de la Bourse on March 23.  23 photos: World reacts to Brussels bombings  Teresa Mancheno, a maintenance worker at Newark Liberty International Airport, attends a vigil in Newark, New Jersey, on March 23.  23 photos: World reacts to Brussels bombings  European Union flags fly at half-staff outside the European Commission in Brussels on March 23.  23 photos: World reacts to Brussels bombings  A young girl lights a candle at the Place de la Bourse in Brussels on Tuesday, March 22.  23 photos: World reacts to Brussels bombings  An image of the Belgian flag is displayed on the Trevi Fountain in Rome on March 22.  23 photos: World reacts to Brussels bombings  The Eiffel Tower is lit up with the colors of the Belgian flag on March 22.  23 photos: World reacts to Brussels bombings  Servicemen with Azov, a Ukrainian volunteer battalion, hold torches during a tribute ceremony at the Belgian Embassy in Kiev, Ukraine, on March 22.  23 photos: World reacts to Brussels bombings  U.S. President Barack Obama and his family observe a moment of silence as they attend a baseball game in Havana, Cuba, with Cuban President Raul Castro, right, on March 22.  23 photos: World reacts to Brussels bombings  A woman reads messages written on the ground at Brussels' Place de la Bourse on March 22.  23 photos: World reacts to Brussels bombings  A New York City church holds Mass for victims of the Brussels attacks on March 22.  23 photos: World reacts to Brussels bombings  A man places flowers outside the Belgian Embassy in Moscow on March 22.  23 photos: World reacts to Brussels bombings  A man looks at flowers and messages outside the stock exchange in Brussels on March 22.  23 photos: World reacts to Brussels bombings  Democratic presidential candidate Hillary Clinton, center, stands for a moment of silence during a roundtable with tribal leaders in Puyallup, Washington, on March 22.  23 photos: World reacts to Brussels bombings  A woman leaves a bouquet of flowers at the base of the Belgium and European Union flags, which were flying at half-staff March 22 at the Belgian Embassy in Washington.  23 photos: World reacts to Brussels bombings  Activists in Multan, Pakistan, condemn the Brussels attack on March 22.  23 photos: World reacts to Brussels bombings  A woman lays flowers at the steps of the Belgian Embassy in Berlin on March 22.  23 photos: World reacts to Brussels bombings  A boy at a makeshift migrant camp shows support for the victims near the village of Idomeni, Greece, on March 22.  23 photos: World reacts to Brussels bombings  Mayors of Istanbul districts walk with consuls from various countries, including Belgium, during a protest condemning terrorism on March 22.  23 photos: World reacts to Brussels bombings  People in Turin, Italy, take part in a rally to remember the victims on March 22.  23 photos: World reacts to Brussels bombings  France's Parliament observes a minute of silence on March 22.  23 photos: World reacts to Brussels bombings  Members of Quebec's National Assembly have a moment of silence on March 22.
So far only a few of the small number of people who left the U.S. to fight in Syria have returned, write Peter Bergen and David Sterman