The police block off a street near the place where explosions happened in Boston, the United States, April 15, 2013. Three people were killed and 144 people wounded in the explosions according to the statistics published on April 16. (Xinhua/Wang Lei)
Twin blasts just seconds apart killed three people, including an 8-year-old boy, and injured more than 140 others at the Boston Marathon.
The explosions drew widespread international condemnation and prompted many major cities around the world to tighten security at a variety of sports events.
The twin bombs exploded at around 3 p.m. local time (1900 GMT) Monday within seconds of each other near the finish line of one of the world's most revered races.
The blasts killed an 8-year-old boy, and injured spectators and athletes at the scene, including a Chinese overseas student and some South African runners.
The Chinese student, who was reported to be in a coma, is studying at Boston University. The student was among 87 Chinese nationals, including 17 women, who were registered to run in the annual race.
South Africa said two of 28 South African runners who had registered for the race were wounded in the bombings.
President Barack Obama said the U.S. government has not yet got "all answers" about the explosions, but vowed to hold anyone behind the bombings accountable.
Leslie Voight, who was in Boston to attend a convention, was taken aback by the bombings.
"There's a lot of sadness on a day that's supposed to be very happy and momentous for a lot of people," Voight said.
Mark Spitzer, who ran in the race, said later that he was "shocked" by the day's events.
"I am shocked somebody would make a statement like this in a wonderful event like Boston Marathon," Spitzer said.
As a precaution after the bombings, cities in the U.S. and other countries beefed up security at hotels, important locations, and at sports events.
A heavy police force remained on duty near the bomb site in Boston hours after the attack.
"You can't walk 30 yards without seeing a police officer," Voight said. "It's just pretty massive."
New York, among other cities, tightened security at hotels and other prominent locations through the deployment of the police department's critical response vehicles.
Police in Chicago began making checks on buildings and sites considered possible high-priority targets.
In Los Angeles, police planed to increase their presence at all scheduled sports events, including the deployment of bomb squads, dogs and other precautions.
Meanwhile, police in San Francisco said they were on heightened alert and urged people to call 911 if they saw anything suspicious.
After the explosions, Rio Olympic organizers immediately reiterated that security is a priority, while Sydney said it will review its safety procedures for the Sydney Morning Herald Half Marathon on May 19.
