"There are people who are really, really bloody," said Laura McLean, a runner from Toronto, who was in the medical tent being treated for dehydration when she was pulled out to make room for victims.

About four hours into the race and two hours after the men's winner crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another explosion could be heard a few seconds later.

By that point, more than 17,000 of the runners had finished the race, but thousands of others were farther back along the course.

The Boston Police Department said two people were killed. Hospitals reported at least 73 injured, at least eight of them critically.

A senior US intelligence official said the two other explosive devices found nearby were being dismantled. The official spoke on condition of anonymity because he was not authorized to discuss the findings publicly.

A woman who was a few feet from the second bomb, Brighid Wall, 35, of Duxbury, said that when it exploded, runners and spectators froze, unsure of what to do. Her husband threw their children to the ground, lay on top of them and another man lay on top of them and said, "Don't get up, don't get up."

After a minute or so without another explosion, Wall said, she and her family headed to a Starbucks and out the back door through an alley. Around them, the windows off the bars and restaurants were blown out.

She said she saw six to eight people bleeding profusely, including one man who was kneeling, dazed, with blood coming down his head. Another person was on the ground covered in blood and not moving.

"My ears are zinging. Their ears are zinging. It was so forceful. It knocked us to the ground."

Competitors and race volunteers were crying as they fled the chaos. Authorities went onto the course to carry away the injured while race stragglers were rerouted away from the smoking site.

Roupen Bastajian, a 35-year-old state trooper from neighboring Rhode Island, had just finished the race when they put the heat blanket wrap on him and he heard the blasts.

"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated. ... At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."

Smoke rose from the blasts, fluttering through the national flags lining the route. Blood stained the pavement in the popular shopping and tourist area known as the Back Bay.

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At least 2 dead, 100 injured in Boston explosions