BOSTON (AP) -- More than 5,000 runners were still on the Boston Marathon course when the bombs went off on Boylston Street.
Race organizers were eager to invite them back -- to let them finish what they started -- and aware of the message that would send.
"The thought was: If those people, like so many others, wanted to have some physical expression of resilience and determination, it would probably be that many of them at least would want to run the whole race," Boston Athletic Association president Tom Grilk said as he prepared for the 118th Boston Marathon. "Can we do that? We thought we'd like to do that."
Then there were the police and firefighters who helped at the site of the explosions; the doctors and nurses and volunteers and EMTs who tended to the wounded; the injured themselves, and friends and relatives who wanted to run in their honor or memory.
After capping the field at 27,000 for about five years, race organizers quickly realized that would not be enough for Monday. But soon the effort to be more inclusive for what proves to be an emotional return to the streets ran into the reality of New England life: The roads built in a horse and buggy era weren't made for tens of thousands of runners, nor the thousands of fans who cheer them on.
"The streets, the roads are pretty much the same roads that were there in 1897. The starting line is still 39 feet wide," Grilk said last week in an interview at the B.A.A. offices, less than two blocks from the finish line. "So we can't create more space. We can use a little more time, so we can stretch it out."
To fit the additional runners -- for a total field of 36,000 that will be the second-largest in the race's history -- organizers will rely again on a staggered start. From 10 to 11:25 a.m. on Monday, four waves of about 9,000 apiece will leave Hopkinton for the 26.2-mile trek to Boston's Copley Square.
Grilk said the expansion was discussed with the eight cities and towns along the route, and all agreed that a smaller race this year was not an option.
"Anything that looked like a reaction that showed fear or in any way giving in to the acts of cowardly terrorists would have been very unfavorably received," he said. "And while there will be pressure and challenge for everybody, everybody welcomes the opportunity to rise to that challenge."
For decades, the field size of the Boston Marathon was limited by the sheer challenge of the distance. What started in Boston in 1897 with 18 men -- and they were all men -- grew slowly at first, still going off with fewer than 200 runners as late as 1960. But the milestones fell quickly after that: more than 1,000 in 1968 and nearly 8,000 by 1979.
For the 100th edition of the race, in 1996, there were 38,708 entrants -- at the time, the largest marathon in history.
And with the crowds came new problems.
The motto for Hopkinton is "It all starts here," but the fact that the town of less than 15,000 happened to be about 26 miles from Boston has not always been considered good luck. As the marathon grew through the years, residents griped about the runners relieving themselves on their lawns or leaving behind piles of granola bar wrappers and empty water bottles when they departed for the Back Bay.
Now, instead of 18 men hitching rides to the start, a fleet of buses shuttles runners to Hopkinton. Portable toilets are trucked in and stationed along the route. Hundreds of volunteers are needed to hand out water, warming blankets and medals. Security, which for a century remained in the background, became a bigger issue after the Sept. 11 attacks and a primary concern in 2014.
And, to squeeze all those runners over a 39-foot starting line -- in the 100th race it took 31 minutes for the field to cross -- the field was broken up into sections: two waves in 2006, and three starting in '11.
Once the B.A.A. decided to invite back the runners who were stopped on the course, another full wave made sense.