Officials prepare for Boston Marathon, no credible threat at this time

Security officials are preparing for the Boston Marathon. 

A helicopter flew low over the Marathon route on Wednesday, measuring naturally-occurring radiation. 

The chopper is from the National Nuclear Security Administration is equipped with radiation sensing technology and its purpose is to measure naturally occurring background radiation in the Boston area.

Measuring naturally occurring radiation helps establish baseline levels and is a normal part of security and emergency preparedness.

Officials discussed security preparations ahead of the Marathon on Wednesday. 

"This year especially we are paying close attention to the recent events that happened in Paris, in Brussels and San Bernardino, we are closely working and sharing information with the FBI and our intelligence community locally and internationally," Mayor Marty Walsh said. "I want to be clear though, at this time there is no credible threat to this event, at all, in the city."

Walsh said delays and detours in the city ahead of the Marathon will be put in place this weekend. 

Marathon attendees will be subject to security checkpoints, and Walsh urged visitors to leave backpacks and drones at home. 

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