This year's race, drawing millions across the five boroughs, had the tightest security ever.
In Brooklyn, where runners made their way down a long stretch, roads were blocked and officers lined the street. As the runners went by, they were accompanied by an escort of police vehicles.
Those cheering the runners on say the increased security presence was noted, but didn't dim the experience.
Karen Hester has been coming to watch the race for 10 years and says she's "not in the slightest" bit concerned about being in a crowded space or the threat of a terror attack.
"There's even more people this year," Hester said. "I don't think anyone is touched by any fear."
The security measures included rooftop observation teams, sharpshooters, aviation units, heavy weapons teams along the route, and undercover officers mixed in with the crowd.
The NYPD also turned to a tactic it has used to protect Trump Tower and the Macy's Thanksgiving Day Parade: 16-ton sanitation trucks filled with sand.
The trucks, along with "blocker cars," were positioned at key intersections to try and prevent anyone from driving onto the course.
About 50,000 runners hit the streets with a crowd of two and a half million cheering them on.
Security for the race has been increasing since the Boston Marathon bombing in 2013.
Vehicle attacks like the one that killed eight people in Lower Manhattan have become much more common around the world.
Police said they would use more of the blocker vehicles for the marathon than they've ever used for any other event.
City officials say everyone taking part in the race should feel safe.
"It will be an extraordinary event as it always is, it will be well protected as it always is and we will take additional measures to ensure that," said Mayor Bill de Blasio.
In addition to regular uniformed officers, there were also NYPD officers in civilian attire "mixing in with the crowds to detect any suspicious activity," NYPD Chief Carlos Gomez said.
Gov. Andrew Cuomo said the state police, the National Guard, the state Office of Emergency Management and other agencies provided added security. Cuomo said the state police doubled the number of troopers posted at high-profile locations, including Kennedy and LaGuardia airports.
The Joint Special Event Threat Assessment from the FBI, Homeland Security and the NYPD said there was no information to indicate a specific, credible threat to, or associated with the marathon.
"We remain concerned about international terrorists and domestic extremists potentially targeting the event," the assessment said. "Unaffiliated lone offenders and homegrown violent extremists are of particular concern."
(The Associated Press contributed to this report.)
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