Think we spotted one here from the Wellesley Square train station

The U.S. Department of Energy’s National Nuclear Security Administration is conducting low-altitude helicopter flights (see video) over Wellesley and other sections of the Boston Marathon route through Monday, April 18 in preparation for the race.

The Nuclear Emergency Support Team aircraft will measure expected background radiation as part of standard preparations to protect public health and safety on the day of an event.

The public may see a twin-engine Bell 412 helicopter, which is equipped with radiation-sensing technology. The helicopter will fly in a grid pattern over the areas at 150 feet (or higher) above the ground at a speed of approximately 80 mph. Flyovers will occur only during daylight hours and are estimated to take approximately two hours to complete per area.