Federal authorities have dispatched a helicopter to fly over Boston measuring radiation levels ahead of the annual marathon on Monday.
The National Nuclear Security Administration (NNSA) said the helicopter might be seen flying at low altitudes as it surveys radiation levels over 10 square miles, covering the 26.2-mile marathon route.
Analyzing background radiation provides officials with the capability to distinguish between normal levels or possible terrorist activity.
Since Thursday, the helicopter has been combing the city at 150 feet or higher above the ground. Officials will perform tests until Sunday.
More than 35,600 people are registered to run the race.
“The measurement of naturally occurring radiation to establish baseline levels is a normal part of security and emergency preparedness,” the NNSA said in a release.
It said it wanted to make the public aware of the low-flying aircraft to ensure the public is not alarmed.
The agency, housed under the Energy Department, is responsible for enhancing national security using nuclear science.
Dzhokhar Tsarnaev has pleaded not guilty to a slew of charges stemming from last year's bombing. Attorney General Eric Holder has directed prosecutors to seek the death penalty in the case.