On the day after two explosions killed three people and injured 176, police presence was heavy and streets remained closed around the area of the Boston Marathon finish line.

The explosions, which occured within seconds of each other at about 2:50 p.m. Monday, happened along crowded Boylston Street, where the marathon finishes.

Witnesses described a bloody, gruesome scene along the street. Smoke billowed from buildings, people were knocked off their feet, and severed limbs were seen.

One of the victims killed was an 8-year-old boy, Martin Richard, of Dorchester, Massachusetts. Another has been identified as Krystle Campbell, 29.

On Tuesday morning, heavily armed officers patrolled the area around Copley Square, with many of the roads blocked.

No arrests had been made in connection with the blasts.

In a briefing Tuesday morning, Massachusetts Gov. Deval Patrick said no other explosives were found. He said the FBI has taken charge of the investigation.

Boston mayor Thomas Menino thanked first responders for their work in the aftermath of the blasts, including “the runners who helped us yesterday in our time of need.”

Officials encouraged anyone with photos or video of the area around the time of the explosions to share them with the FBI. The FBI's hotline is 1-800-CALL-FBI; the email address is boston@ic.fbi.gov.

The Associated Press, citing unidentified sources close to the investiagtion, reported the two bombs were made up of pressure cookers, one packed with ball bearings and the other with shards of metal.

Runners were seen heading out of hotels Tuesday morning, many sharing their stories of where they were when the blasts occurred.

On Monday, Boston Police Commissioner Ed Davis urged people to stay home, return to their hotel rooms, and not congregate in large crowds.

As dusk fell, police presence was heavy in the city, especially in the Copley Square area. SWAT officers in full fatigues and armed with assault rifles stood guard at hotels.

Families trying to connect with runners or other loved ones were asked to call 617-635-4500. People who have information that could lead to an arrest were to call 1-800-494-TIPS.

"We don't know who did this or why," said President Obama. "But make no mistake we will get to the bottom on this and we'll find out why they did this."

"Today is a holiday in Massachusetts – it's Patriots' Day. It's a day that celebrates the free and fiercely independent spirit that this great American city of Boston has reflected since the earliest days of our nation. And it's a day that draws the world to Boston streets in the spirit of friendly competition. Boston is a tough and resilient town and so are its people," the president said.

Brian Chung, 21, a Boston College student from Long Island, was running with Claire Townley, 20, also a B.C. student, from Omaha, Nebraska.

They had just finished the race, him coming in at 3:45 and she in 3:50, about five minutes before the explosions.

“We were both mentally and physically tired at that point and all of a sudden we heard an explosion and at that point it didn’t matter how tired we were,” said Townley.

“We’re lucky to be alive,” he said.

They were waiting for Townley's father, who was running behind them.

Townley didn’t know where he was and only found out an hour later he was alive.

Garrett Hill, 37, of New York City, was in the medical tent getting treatment after finishing the race when he heard the explosion. He then saw victims being brought into the tent, some in desperate need of attention, including one person naked on a stretcher.

“It puts things in perspective,” Hill said. “For the past 6 months I’ve been training for this … I usually take a photo of my abs at the finish line and this makes me realize how frivolous I can be.”

Of the 23,326 runners who started the race, 17,584 crossed the finish line, the Boston Athletic Association said. The last recorded finisher crossed the line at 2:57 p.m.

“There were two bombs that exploded near the finish line in today's Boston Marathon,” the B.A.A. said. “We are working with law enforcement to understand what exactly has happened.”

The blasts were heard and felt inside the press center, located at the Fairmont Copley Plaza Hotel near the finish line.

Moments later a spokesman came into the room and told reporters the room was on lockdown and that no one was allowed in or out.

Photos showed smoke coming from a building on Boylston Street. Windows were blown out of at least one store on Boylston.

Medical tents that were set up for the marathon were full of people injured in the blasts, according to reports.

The finish line area was cleared of spectators and runners.

Runners who were still on the course when the explosions happened were diverted from the course.

Runner's World editor-at-large Amby Burfoot, running on the 45th anniversary of his Boston victory, was about 3/4 of a mile from the finish at the time of the explosion.

"We were about to go under the Mass Ave underpass when suddenly there was congestion in front of us," Burfoot said in an email. "Soon it became clear it was runners congesting."

"Everyone's cell phones starting going off. I got a call from [my wife] Cristina that the finish line was cleared and the race was over."

Burfoot walked back to his hotel.

RW Executive Editor Tish Hamilton was also nearing the finish line when the explosion happened.

“I was on Boylston, in front of Lord and Taylor. I saw the first explosion go off. The cops turned to look at it, and then the second one went off," Hamilton said in an email. "All the spectators and people on the course turned and ran the opposite direction. I didn’t finish – I went straight into the mall.”

RW Brand Editor Warren Greene was at an industry event near the finish line.

"The blasts went off very close to each other. It shook the floor. We had no idea what was going on and then we heard the 2nd blast," Greene said. "The barricades fell away and folks started running every which way. The cops came in and shut the finish area down. The race stopped. Runners just stopped running and looked at finish."

Erin Strout and Charlie Butler contributed to this report.

Scott Douglas Scott is a veteran running, fitness, and health journalist who has held senior editorial positions at Runner’s World and Running Times.

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