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Some spectators took their eyes off the road and turned their attention to the sky before Monday's race when four HH-60M Air Ambulance Blackhawk Helicopters with the Massachusetts Army National Guard flew over the Boston Marathon route.

Approximately 750 soldiers and airmen from the Massachusetts National Guard are on duty providing law enforcement, medical assistance, explosive ordnance disposal capability and transportation support, a U.S. military official told NBC News.

“We know of no specific credible threats to this event, but we remain vigilant,” said U.S. Secretary of Homeland Security Jeh Johnson at a 10:15 a.m. news conference at the Massachusetts Emergency Marathon Agency headquarters.