The FBI says the two bombs that exploded Monday at the Boston Marathon were carried in a dark-colored bag, and possibly consisted of pressure cookers packed with nails and ball bearings.
FBI examiners are looking at fragments recovered from the scene, as they try to piece together how the attack happened.
Authorities do not have any suspects, and are asking the public to send in any photos or videos taken along the marathon route to help find who was responsible.
Watch related video report by VOA's Richard Green
VOA correspondent Carolyn Presutti, reporting in Boston, says police are also asking people to call in any possible tips.
Deadly bombings in the United States
April 15, 2013: Twin blasts at the Boston Marathon kill at least 3, injure more than 140
September 11, 2001: Hijacked jets crash into World Trade Center, Pentagon and a Pennsylvania field killing nearly 3,000
July 27, 1996: Atlanta Summer Olympics bombing kills 2, injures more than 100
April 19, 1995: Car bomb at Oklahoma City federal building kills 168, injures more than 500
February 26, 1993: Van explosion in World Trade Center garage kills 6, injures more than 1,000
December 29, 1975: Bomb at New York's LaGuardia Airport kills 11, injures 75
September 16, 1920: Bombing in New York's Wall Street area kills 40, injures hundreds
"'Somebody knows something' was the last thing I heard from one of the law enforcement officials. They said if any person that you know mentioned the date of the marathon with the intent of doing something wrong on that day, let us know," said. "They have received more than 2,000 tips, and the police say they are not letting any of those tips be ignored. They are paying attention to all those tips, following up on all those tips in hopes that they get the break they need in this case."
Related video report by VOA's Jeff Seldin:
Police and firefighters unions in Boston have offered a $50,000 reward for information leading to an arrest.
The FBI agent in charge of the case says investigators will "go to the ends of the Earth" to identify who carried out the bombings
Maryland Congressman Dutch Ruppersberger said after a security briefing Tuesday it is not likely that al-Qaida or any foreign government was involved because of the lack of prior intelligence.
Two blasts seconds apart killed three people at the finish line of the Boston Marathon, including an eight-year-old boy. One-hundred-76 people were injured. A number of them lost limbs.
U.S. President Barack Obama called it a "heinous and cowardly act of terrorism." He ordered American flags to be flown at half-staff in honor of the victims. The president will attend an interfaith service in Boston for the victims on Thursday.
Getting back to normal
VOA's Presutti said more people were out on the streets of Boston on Tuesday, with memorials springing up at barricades along the race route as they city tries to heal.
"The memorials consisted of flowers, t-shirts, notes, some clothing, and the one thing that struck me at this one memorial that we saw was a framed medal. It was a medal that someone had been given for finishing the Boston Marathon, and they put this medal in a frame, and they put a really nice saying next to it. It said something to the effect of, 'I finished the race, but I know a lot of you didn't get to see your loved ones finish the race,' or 'You didn't get finish the race so I give you my medal because I already had my congratulations at my finish line," she said.
Presutti also described the city Tuesday night lit up in various spots in support of those injured and killed. The displays included a bridge featuring yellow and blue lights to match the marathon's colors and a building at the Massachusetts Institute of Technology glowing with the red, white and blue of the American flag.
A doctor at Massachusetts General Hospital said Tuesday those victims listed as critical are improving. He called the wounded "amazing people" and that he was touched to hear victims who lost a leg say how glad they are just to be alive.
Cities worldwide stepped up security following the explosions. In Britain, police said they are reviewing security plans for Sunday's London Marathon, the next major international marathon.
Marathon winner reacts
The blasts in Boston took place about four hours into the race, long after the marathon winners had finished, but when many of the slower runners were crossing the finish line with family members and friends waiting to congratulate them. More than 23,000 runners from around the world competed.
Ethiopian athlete Lelisa Desisa was the winner.
“I feel very sorry about the lives lost in these senseless explosives attacks. All of us runners had gone back to our hotel rooms to get ready to catch a flight back to Washington when we heard the TV news about the blasts," he said. " What happened yesterday [Monday] will not keep me from entering future marathons. It’s not going to intimidate me.”