The agency, part of the Department of Energy, says it is measuring background radiation "to protect public health and safety" on the day of the Marathon. That means a U.S. government Bell 412 helicopter will be flying low, around 150 feet over the marathon route in a grid pattern at about 80 mph.
NEST — the team running the flights — specializes in finding and eliminating nuclear weapons.
The NNSA does similar fly-bys before every Boston Marathon. The agency said it was just letting the public know to make sure no one is alarmed.