The FBI has joined Massachusetts law enforcement agencies in investigating the theft of Boston Police Department uniforms, which were stolen from a Weymouth business, just weeks before the city’s annual marathon is set to kick off.

Friday marked nine years since the Boston Marathon Bombing of 2013, when three people were killed and more than 260 were hurt. Ahead of Monday’s 126th Boston Marathon, Boston Police Superintendent-In-Chief Greg Long is urging attendees not to hesitate to report someone acting suspicious, even if they are dressed in a uniform.

The Weymouth Police Department announced earlier this week that it was working with the FBI in investigating the theft of several Boston Police Department uniforms that were stolen from a local uniform supply store.

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Investigators believe the uniforms were stolen between 3:30 p.m. March 27 and 9:30 a.m. March 28, the department said Wednesday. Authorities are now offering a reward of up to $5,000 for information leading them to their suspects.

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Shortly after the announcement in a Wednesday Facebook post, some social media users began expressing concerns about how the stolen uniforms might be used.

"I hope these uniforms won’t be used at the Marathon," one person wrote.

During a press conference about safety precautions ahead of Marathon Monday, Boston Police Superintendent-in-Chief Long was asked about how a person should respond if they have concerns about someone who they think is "posing as somebody else."

"I think people have a keen understanding of what kind of behavior is normal, what’s not, what’s a little bit of," Long said. "If you see someone, whether it’s dressed in a uniform, has a vest on, that you think might be a worker, but they’re acting a little bit odd, a little bit suspicious, a little bit out of the norm for what you’d expect someone to be doing, I’d urge anybody not to be afraid to call 911."

He added: "I’d rather err on the side of caution than not."

Police officers stand watch as Boston Marathon runners race along the course, April 20, 2015, in Boston. (AP Photo/Robert F. Bukaty)

Monday will mark the first time the Boston Marathon will be held on its regularly scheduled Patriots Day date following postponements and changes as a result of the coronavirus pandemic.

Approximately 30,000 people are set to run in the marathon, officials have said.

Long said there was no known credible threat to the race, but said spectators and runners could expect heavy security, bag checks, camera surveillance, police observation points and the presence of officers in plain clothes and in uniform.

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He urged anyone to call 911 or notify a local police officer "if you see something, no matter how small, something that seems not right or something out of the ordinary."

Fox News' Haley Chi-Sing contributed to this report.