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People light candles at a memorial set up outside the stock exchange in Brussels on Tuesday, March 22, 2016. Explosions, at least one likely caused by a suicide bomber, rocked the Brussels airport and subway system Tuesday, prompting a lockdown of the Belgian capital and heightened security across Europe. (AP Photo/Geert Vanden Wijngaert)

People light candles at a memorial set up outside the stock exchange in Brussels on Tuesday, March 22, 2016. Explosions, at least one likely caused by a suicide bomber, rocked the Brussels airport and subway system Tuesday, prompting a lockdown of the Belgian capital and heightened security across Europe. (AP Photo/Geert Vanden Wijngaert)