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.
"'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.
In this image from video provided by WBZ-TV, spectators and runners run from what was described as twin explosions that shook the finish line of the Boston Marathon, April 15, 2013.
An emergency responder and volunteers, including Carlos Arredondo in the cowboy hat, push Jeff Bauman in a wheel chair after he was injured in an explosion near the finish line of the Boston Marathon April 15, 2013.
Medical workers transport the injured across the finish line during the 2013 Boston Marathon following an explosion, April 15, 2013.
Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion, April 15, 2013.
One of the blast sites on Boylston Street near the finish line of the 2013 Boston Marathon is investigated by two people in protective suits in the wake of two blasts in Boston April 15, 2013.
Runner John Ounao cries when he finds friends after several explosions rocked the finish of the Boston Marathon, April 15, 2013.
A police officer clears Boylston Street following an explosion at the finish line of the 2013 Boston Marathon, April 15, 2013.
Medical workers aid a wounded woman at the finish line of the 2013 Boston Marathon following two explosions there, April 15, 2013.
Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, April 15, 2013.
A woman is comforted by a man near a triage tent set up after explosions went off at the 117th Boston Marathon, April 15, 2013.
A Massachusetts state police officer guards the area containing the medical tent, rear, following an explosion at the 2013 Boston Marathon, April 15, 2013.
An unidentified Boston Marathon runner leaves the course crying near Copley Square following an explosion, April 15, 2013.
A Boston police officer wheels in injured boy down Boylston Street as medical workers carry an injured runner following an explosion during the 2013 Boston Marathon, April 15, 2013.
Justine Franco of Montpelier, Vermont, holds up a sign near Copley Square looking for her missing friend, April, who was running in her first Boston Marathon, April 15, 2013.
President Barack Obama leaves the podium after speaking in the press briefing room at the White House, April 15, 2013, following the explosions at the Boston Marathon.
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.”