The FBI says the investigation of Monday's deadly bombing at the Boston Marathon will be "worldwide."
Special agent in charge Rick DesLauriers says authorities will "go to the ends of the earth" to identify those responsible for what he called a "despicable crime."
The FBI is leading the effort to investigate what a White House official said was "clearly an act of terror."
Three people were killed in the blasts, including an eight-year-old boy. Boston Police Commissioner Ed Davis said 176 people were injured, 17 critically. A number of victims lost limbs. A doctor said one of the victims was maimed by what looked like ball bearings or BBs.
Massachusetts Governor Deval Patrick confirmed only two explosive devices were found, the two that exploded. Officials say there are no known additional threats.
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.
No person or group has claimed responsibility for the blasts, and at a news conference Tuesday officials refused to comment on reports of suspects in custody. President Barack Obama has ordered American flags to be flown at half-staff in honor of the victims.
FBI agents searched an apartment in the Boston suburb of Revere overnight and have appealed to the public for amateur video and photos that might yield clues to who carried out the bombing.
Location of the marathon finish line in Boston, Massachusetts, where two deadly explosions occurred.
The blasts took place about four hours into the race, long after the winners had finished, but at the time a high number of runners and their supporters are usually around the finish line area. The competition, which attracted more than 23,000 runners from around the world, was halted after the bombs went off.
VOA correspondent Carolyn Presutti said early Tuesday investigators were working in a wide area around the bombing scene. She said she saw a lot of security around the city, but otherwise no one on the streets.
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.
New York City officials said police have increased security in the city, including near prominent hotels, in response to the blast. Washington, D.C. and Los Angeles also are on a heightened state of alert.
Azeem Khan, a Pakistani-American who was three miles from the race finish at the time of the explosions, told VOA's Urdu service that up until that point it had been a "joyful day."
"Running the race, it was such an amazing experience. The joy of the people is what helped me keep going when I was so tired," Khan confided, "and how everyone was out, the entire state of Massachusetts was out. Little kids -- even if they weren't part of the marathon staff -- little kids hanging around with Dixie cups and people who baked cookies for us, and people were handing out food and telling us to keep going," he recalled, "and giving us handshakes as we were running. It was just such a joyful day, and to turn such a joyful day into massacre like this, just can't help you but feel anything but anger."
Khan said as bad as Monday was, he will run another big city marathon. He plans to sign up for the Chicago Marathon in October.