If you live along or near the Boston Marathon route and have been seeing helicopters flying around this week, don’t worry, the flyovers are part of a routine security measure.

The U.S. Department of Energy’s National Nuclear Security Administration has been conducting low-altitude helicopter flights over the route of the Boston Marathon since Tuesday, the department said in a news release.

The twin-engine Bell 412 helicopters are measuring naturally occurring background radiation, the department said, and will finish measuring on Friday.

The helicopters fly in a grid pattern over the area at 150 feet or higher above the ground at a speed of approximately 80 miles per hour, the department said. Flyovers will occur only during daylight hours.

The measurement of naturally occurring radiation to establish baseline levels is a normal part of security and emergency preparedness for major public events, the department said.

The department has been conducting these measurement flights before the Boston Marathon since 2014, the year after the Boston Marathon bombing.

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