BOSTON, Massachusetts (Reuters) - When runners and spectators turn out for this year's Boston Marathon on Monday (April 18), they'll see plenty of security, working both in plain sight and behind the scenes.

Working to secure the route on the day of the race are 5,000 uniformed and undercover officers from multiple agencies.

Officials will monitor activity on the streets from an underground command bunker. They'll also keep a close eye for any suspicious online chatter.

"We are working with the FBI, we are working with all our partners to check on whoever might be a threat. We have our eyes on certain people and what their travels might be," said Boston Police Commissioner Bill Evans.

Police helicopters will also be in the sky to help with surveillance.

It's been three years since bombs ripped through the finish line killing three and injuring hundreds.

Since then, security has been top priority. And officials are taking note of the recent terrorist attacks in Europe.

"Obviously with the attacks we've seen around the world this year, it's certainly concerning to everyone but the police are working closely with other agencies, just to make sure any type of threat we investigate nothing to this point," said Boston Mayor Marty Walsh.

Backpacks and drones are banned from this year's race.

The marathon will host 30,000 participants, with another one million spectators expected along the course.