Boston was put on high alert Monday as the FBI took over the hunt for whoever set off two powerful bombs at the finish line of the city's marathon, killing at least three people and wounding at least 140 people in a vicious blizzard of shrapnel, officials said.
"I am not prepared to say we are at ease at this point in time," Boston Police Commissioner Ed Davis said at a briefing six hours after the attack transformed the celebrated race into a blood-spattered crime scene.
The bombs were filled with ball bearings that sprayed into the crowd when they exploded, law enforcement officials told NBC News. The dead included an 8-year-old boy, law enforcement officials said, and some victims lost limbs.
At an evening briefing, officials said the National Guard had cordoned off the area to preserve evidence. Police were planning to be out in force at daybreak. Commuters were warned their bags would be searched.
Long after nightfall, investigators were inspecting the mounds of personal belongings left by those fleeing to scene to make sure there were no more explosives, while firefighters were chasing bomb scares around the jittery city. SWAT team members were on patrol at several hotels in the Back Bay area.
Water cannons were used to clear a number of suspicious packages near the scene. A fire at the John F. Kennedy presidential library more than an hour after the blasts appeared to simply be caused by an electrical short, police said.
Massachusetts Gov. Patrick Deval said only the streets around the bombing site would be on lockdown but he urged everyone to be vigilant.
"It will not be business as usual," he said. "It's not going to be easy, simple or regular."
Boston Police Commissioner Ed Davis confirms that there is no suspect at Brigham and Women's Hospital as previously reported and promises they will 'turn every rock over to find the people who are responsible' for deadly blast at Boston Marathon.
The FBI said it was flooding the city with investigators from the Joint Terrorism Task Force and declined to say if agents had any suspects or clues to a motive.
"It is a criminal investigation that is a potential terrorist investigation," said Special Agent in Charge Richard DesLauriers.
Related: Slideshow of scene of explosions
Boston police issued an alert for a rental van that may have sought access to the marathon route, and another alert for a man wearing dark clothing and a hood who was seen leaving the scene of the blasts.
But as he revealed the death toll had climbed to three, Police Commissioner Ed Davis stressed that authorities had not identified a suspect and were instead "talking to" numerous people.
"This cowardly act will not be taken in stride," Davis said. "We will turn every rock over to find out who is responsible for this."
His comments echoed President Obama's vow earlier in the day to bring the attacker or attackers to justice.
"We still do not know who did this or why, and people shouldn’t jump to conclusions before we have all the facts, but make no mistake, we will get to the bottom of this," Obama said from the White House several hours after the blasts.
Related: Obama pledges federal help