BOSTON --On the third anniversary ofthe Boston Marathon bombings, the city fell silent Friday at 2:49 p.m., the moment the first of two bombs went off, killing three people and wounding 264 others.
This year's marathon is Monday. Boston's top cop wanted to compete, but he'll be too busy.
The ISIS attacks in Brussels last month has given Boston Police Commissioner William Evans a lot to think about.
"It just sort of brought back a lot of emotions of what happened here," Evans told CBS News. "We've got to stay focused on the race and make sure it goes off safely."
Five thousand uniformed officers from eight cities will join the National Guard, state police, and scores of federal agents from the Secret Service and FBI to secure the marathon route.
They will be directed from an underground emergency command bunker. In the intelligence sharing room, agents monitor online chatter.
Evans said the threat of copycat attacks is a concern.
"We're working with the FBI, we work with all our partners to make sure we pay attention to who might be a threat to the Marathon," Evans said. "We have our eyes on certain people, as far as what their travels might be."
There will be 30,000 people running the 26.2-mile route and one million spectators along the way. Backpacks have been banned and so have drones. There are drone detectors to alert authorities.
Four Massachusetts State Police helicopters will be in the sky providing surveillance help. Infrared cameras can scan for people on rooftops and zoom in on suspicious objects like unattended bags.
But no amount of preparation will be enough for Commissioner Evans to let down his guard. He said vigilance is key.
"We have a lot of undercover officers working the crowd, we have bomb-sniffing dogs," Evans said. "I'm pretty confident we're going to have a great race, but any marathon -- when you cover that amount of distance, unfortunately, you can never say the whole route is secure."
Evans said there is no credible or specific threat against the marathon. But still, it will be all hands on deck staffing until the last runner crosses the finish line and the crowds disperse.