"This was a heinous and cowardly act," U.S. President Barack Obama said at the White House.
"Any time bombs are used to target innocent civilians, it is an act of terror. What we don't yet know, however, is who carried out this attack."
Three people were killed and more than 170 injured at the finish line of the marathon, some with horrific injuries, after two bombs were detonated about 10 seconds and 90 metres apart. Many of the race's 23,000 participants had already crossed the finish line, but thousands more were still approaching.
Among those killed was an eight-year-old boy who was watching the race with his family.
Young Martin Richard's mother and six-year-old sister were gravely injured. A photo taken a year ago of the smiling little boy holding a sign that read: "No more hurting people" went viral on Tuesday.
The explosives were placed in six-litre kitchen pressure cookers packed with ball bearings, nails and scraps of metal to inflict maximum damage. They were stuffed into black duffel bags and left on the ground, The Associated Press reported, citing law enforcement sources.
Later in the day, Richard DesLauriers, the special agent in charge of the FBI's Boston division, confirmed those details.
Investigators began recovering evidence from the bombing site on Tuesday morning, he told a news conference.
"Among items partially recovered are pieces of black nylon, which could be from a backpack, and what appear to be fragments of BBs and nails, possibly contained in a pressure cooker device," he said.
"In addition, this morning it was determined that both of the explosives were placed in a dark-colored nylon bag or backpack. The bag would have been heavy because of the components believed to be in it."
Pressure-cooker bombs have been used in attacks in Afghanistan, India, Nepal and Pakistan. And in 2010, authorities found a pressure cooker stuffed with 120 firecrackers — among other explosives — in the vehicle of Faisal Shahzad, an American citizen who tried to detonate a car bomb in New York's Times Square.
Instructions for making bombs with pressure-cookers are also readily available online in the United States.
The Boston Marathon bombs were crudely designed devices that had a devastating impact — several of the wounded had their limbs blown off in the blasts, including one man who was watching his girlfriend cross the finish line. A photo of Jeff Bauman Jr. being wheeled to paramedics was one of the most shocking images to emerge from the carnage.
Federal investigators were searching for those responsible as Americans were once again left reeling from scenes of blood-soaked chaos and fearful for their own safety.
But the FBI assured Americans that there were no known additional threats. Boston officials, meantime, said they'd conducted two security sweeps of the route before the race.
And yet in D.C., a letter addressed to a Mississippi senator was intercepted at the Capitol building's off-site mail facility. It tested positive for the deadly poison ricin.
Harry Reid, Senate majority leader, said it was sent to Republican Sen. Roger Wicker. Authorities said the letter had a Memphis postmark.
The Boston bombings occurred on what's known as Patriots Day in Massachusetts. It commemorates the opening shots of the American Revolution at Concord and Lexington, northwest of Boston, in 1775.
DesLauriers said the investigation is "in its infancy," adding there were no claims of responsibility.
Investigators were poring over surveillance camera images from a substantial number of videos obtained from businesses in the downtown area where the explosions occurred, said Edward Davis, Boston police commissioner. The FBI was also expected to issue subpoenas for records from cell towers in the area.
"This is probably one of the most photographed areas in the country yesterday," Davis said.
The FBI's Boston division urged people to send along any information, photographs, videos or other details from the bombings.
"No piece of information or detail is too small," it said in a statement. Another FBI official vowed investigators would go to the "ends of the Earth" to apprehend whoever is responsible, adding agents are following a number of leads.
Law enforcement authorities, including bomb experts, searched an apartment in Revere, Mass., a Boston bedroom community northeast of the city, and removed some items from the home on Monday night. They said little publicly, however, about how the search was linked to the bombings except to acknowledge it involved a "person of interest."
An injured Saudi man initially thought to be a suspect was ruled out as one on Tuesday.
Obama said investigators don't know yet whether the bombings were perpetrated by an international organization, a domestic group or a "malevolent individual." But he made clear: "The American people refuse to be terrorized."
From coast to coast, officials were on high alert, tightening security at airports, government buildings, sporting events and landmarks where citizens gather in large numbers.
At Virginia's Dulles International Airport, which serves D.C., however, there were short, swift lineups for international visitors coming through U.S. Customs and Border Protection mid-day Tuesday.
In Canada, authorities said security was ramped up at all border crossings.
"We want to ensure that if someone would be using our borders to cross into Canada for any inappropriate reason, that those individuals are detected," Public Safety Minister Vic Toews said Tuesday.
Intelligence experts said publicly they suspected the attacks were the work of a domestic terrorist rather than al Qaida.
"This happened on Patriots Day — it is also the day Americans are supposed to have their taxes in — and Boston is quite a symbolic city," said Richard Barrett, the former UN co-ordinator for an al-Qaida and Taliban monitoring team who has also worked for British intelligence.
Taliban in Pakistan, which claimed responsibility for the 2010 attempt in Times Square, has denied any role in the Boston Marathon attack.
_ With files from The Associated Press