Security tight for Boston Marathon

Three years after bombs ripped through the finish line, security is being ramped up for the Boston Marathon.

When runners and spectators turn out for this year's Boston Marathon Monday, they'll see plenty of security... working both in plain sight and behind the scenes.

5,000 uniformed and undercover officers from multiple agencies will be working to secure the route on the day of the race.

This underground command bunker is where officials will monitor activity on the streets. They'll also keep a close eye for any suspicious online chatter.

SOUNDBITE: BOSTON POLICE COMMISSIONER BILL EVANS, SAYING:

"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."

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

NATS: "This is the infrared camera right now."

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.

SOUNDBITE: BOSTON MAYOR MARTY WALSH, SAYING:

"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."

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

30,000 people are participating in the marathon with another one million spectators expected along the course.