US leaders including Vice President Joe Biden and survivors of the Boston Marathon bombing shared messages of thanks and defiance at a tribute to the three people killed and 264 wounded in the attack exactly one year ago.
From Patrick Downes, who lost a leg when a pair of homemade bombs ripped through the crowds at the race's finish line, to Biden, speakers recalled how those on the scene, from police officers to spectators, reacted immediately to help the wounded amid the chaos on April 15, 2013.
Former Boston Mayor Thomas Menino, who managed the response to the attack in the final year of his two decades in office, recalled the struggles of the families of Martin Richard, 8, Krystle Campbell, 29, and Chinese national Lu Lingzi, 23, who died in the attack
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Some children, particularly those who have been more directly affected, may show symptoms of stress and trauma.
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The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.