The two explosions at the Boston Marathon forever changed the 26.2 mile courses at marathons held around the world.
"Just looking out those glass windows and the moment that I realized or it hit me that it was a bomb," said Steve Karpas, the managing director of the Houston Marathon Committee.
He was in Boston when the bombs went off.
"It caused us as an industry to look at ourselves and reinvent our security processes. It brought us into an era that made us step up our game if you will," Karpas said.
The explosions in Boston mean more security at the start and finish lines at races, where the crowds are the largest and congregate the longest. Unlike a sports stadium, a marathon stretches more than 26 miles.
"The course is open to the public. And so we can do as much as we can. But ultimately there's only so much we can do," said Karpas.
Karpas and others from Houston plan to run the Boston Marathon on Monday.
"Once the bombing happened it was just so surreal," said Ruth Rosas of Shoreacres.
She had just finished the race when the bombs went off. Her time was fast enough that it qualified her to run the Boston Marathon.
"I'm going back for myself but I'm also going back for those who can't. If I have the opportunity to go back, then I need to think beyond myself. I'm running for those who can't, who can't be there," Rosas said.
Rosas will also run to raise money for 13-year-old Paige Lejeune, of Deer Park, who is fighting T-cell leukemia. Rosas wants to help Paige and her family pay for the expensive treatments.
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Rosas has already raised more than $5,500. The goal is $25,000.
Rosas, Karps and 36,000 others will strengthen "Boston Strong" in the Boston Marathon on Monday.