With both London and Hamburg, Germany, gearing up for marathons on Sunday, and major races in Philadelphia, New York and Chicago on the horizon, the question of how to secure a 26.2-mile stretch through a city's streets to protect runners and spectators looms after the explosions at the Boston Marathon on Monday.
As government and law enforcement officials around the country announced plans to reinforce security measures at strategic locations and review safety plans before major races, best practices were being scrutinized to keep future marathons safe.
"Any public event of that magnitude is very difficult to secure," said Lou Marciani, director of the National Center for Spectator Sports Safety and Security at the University of Southern Mississippi in Hattiesburg, Miss. "There's no perimeter control, there's no access control, there's nothing."
Complete Coverage: Terror at the Boston Marathon
Marciani said marathon security personnel -- which includes local, state and federal authorities, as well as the event management team -- work together nearly a year in advance of the race to minimize security risks.
But monitoring more than 26 miles of a race is arduous, given the difficulty of observing both the goings on within a city and the crowds and participants of a race.
The Boston Marathon had more than 23,000 participants on Monday, while bigger races, like the Chicago Marathon and New York City Marathon, typically have around 45,000 runners.
The crowds, participants and the distance of the courses make it difficult to secure such large, open areas, said Alana Penza, program manager at the Institute for the Study of Sports Incidents at the University of Southern Mississippi.
"You have people that have their backpacks with them, a runner may have a hydration pack on them, another person may be coming to watch the race and they may have a backpack," Penza said. "You see them so much, you stop thinking that any of those bags might have explosives or where the garbages are."
"You can perimeter control the finish line -- that's something all marathons have been doing for a long time. That's one area of the 26 miles that we have some handle on," Marciani said. "I don't know if you can do that for 26 miles. I doubt it."
Penza told ABC News that a marathon presents a more daunting security risk because there are likely to be more vulnerable points along the stretch of the race's course. She said that this may have been the case with the explosions at the Boston Marathon.
"If there is a person that detonated the explosives, if this person was trying to cause mass casualties, they wouldn't be likely to do it in areas where there are a few people coming through," she said. "You look at the areas with the most vulnerability, which is the finish line."
Penza said that the sheer distance of a marathon requires security officials to consider "man-to-man coverage" in areas that are most susceptible to security threats.
But both Penza and Marciani said that regardless of security best practices coming under evaluation, the responses to the attack in Boston were outstanding.