Ashley Liew, Singapore's top finisher at last year's Standard Chartered Marathon Singapore, was already in his hotel room when the explosions at the Boston Marathon took place on Monday.
The 27-year-old finished his race in about 2hr 44min.
He said: "My hotel is a few train stops away from the finish line, but I heard all these sirens and I thought perhaps there had been some medical problems at the race."
Liew, currently studying chiropractic in the United States, has flown out of Boston and is on his way back to South Carolina where he is based.
"The area was still in quite a mess when I was leaving and I had to reschedule my flight because the area was on lockdown and I couldn't make my flight in time."
Yvonne Yeo, 37, had finished her race and was headed to the baggage deposit counter when the explosions took place.
She said: "People were running away from the finish line and it was very chaotic."
Singapore's "Blade Runner" Abdullah Shariff, also took part in the race. The amputee, who runs with the aid of a prosthetic left limb, is understood to be safe back in his hotel.
Acting Minister for Culture, Community and Youth Lawrence Wong also posted a message on his Facebook on the incident.
He said: "Very shocked to hear about the explosions at the Boston marathon. I was in Boston in 2003, and remember participating in the event then (did the half-marathon).
"To runners, the Boston marathon is considered the 'holy grail' of marathons. It's a wonderful, inspiring event - the entire course is full of fans coming together to cheer the runners along, especially near the finish line.
"It saddens me when I think of how many of these spectators have been injured by the blast. My thoughts and prayers are with all those affected by this horrific incident."
There were a total of 18 Singaporean runners at the marathon. It is believed all are unhurt. Three have been killed from the blasts with more than a hundred wounded so far.