"We can't put fences around it; we can't put an officer every 2 feet," he said. "So we rely on every set of eyes out there."
So, as it often does in a free society, it comes down to us, to that word, to vigilance.
"Remain vigilant. You have to," said Mike Brooks, a law enforcement analyst for In Session and HLN and a former detective in the intelligence division for the Washington Metropolitan Police Department.
But security experts worry that down the road, many Americans may weary of maintaining such a high level of cautiousness.
"We as a country tend to lull ourselves into a false sense of security over the passage of time," said Andy Lamprey, vice president of the security firm Andrews International and a former Los Angeles Police Department senior SWAT supervisor.
The interest in the Boston terrorism case "will last for a few days and perhaps a week, and then it will become a distant memory for most people," he said.
It happened after the September 11 attacks. A month after them, nearly six in 10 Americans were worried that they or someone they loved would become a victim of a terrorist attack, according to a Gallup Poll at the time.
By 2011, the number had fallen to a little over one in three -- nearly what it was at its lowest point before the 2001 attacks.
"It's the old story about crying wolf and eventually people turn a blind eye and a deaf ear to it. They get weary of hearing it," Lamprey said. "It's very difficult to remain at that heightened state of awareness. You can't do it all the time."
In Oshkosh, Wisconsin, runners will head out this weekend for a half-marathon. Sure, race director Gloria West told CNN affiliate WBAY-TV, they'll think about security just that much more. Boston will be on their minds.
But they won't be deterred, she told the station.
"We want to send a message," she said. "We can't all go home and stay in our houses."