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.

These tests come a year after two pressure cooker bombs exploded at the Boston Marathon’s finish line, killing three people and wounding more than 260. The anniversary was on Tuesday.

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 HolderEric H. HolderOvernight Tech: Senate moving to kill FCC's internet privacy rules | Bill Gates pushes for foreign aid | Verizon, AT&T pull Google ads | Q&A with IBM's VP for cyber threat intel Uber leadership sticking by CEO Top Dems prep for future while out of the spotlight MORE has directed prosecutors to seek the death penalty in the case.