The US Federal Bureau of Investigation (FBI) worked with local officials through early Tuesday in the Boston metropolitan area to apprehend any persons of interest linked to the explosions at the city's annual marathon, which claimed three lives and injured at least 144 people. No individual or group has yet claimed responsibility for the attack.

As of early Tuesday morning local time, no suspects had been taken into custody.

According to reports in the newspapers "The Boston Globe" and the "Boston Herald," the FBI, Homeland Security and local officials searched an apartment in Revere, Massachusetts, which lies about 5 miles (8 kilometers) northeast of Boston's downtown. Officials would not confirm if the search was related to the explosions.

Officials have not yet determined whether the attack was foreign or domestic, even refraining from calling the twin explosions "terrorism."

Meanwhile, other US metropolitans, such as New York, Washington and Los Angeles and San Francisco, heightened security amid the uncertainty.

'We will get to the bottom of this'

In a televised statement following the attack, US President Barack Obama urged the public to remain calm.

"We still don't know who did this, or why, and people should not jump to conclusions before we have all the facts, but make no mistake, we will get to the bottom of this, " Obama said, adding that Bostonians and the victims had the full support of the US government and the American people.

"Any responsible individuals, any responsible groups will feel the full weight of justice," he said.

An unnamed White House official later released a statement, however, saying it would be treated by the authorities as an act of terror.

"Any event with multiple explosive devices - as this appears to be - is clearly an act of terror, and will be approached as an act of terror," the official said.

Many victims require amputations

Three people died in the blasts, including an 8-year-old reportedly cheering on his father, a marathon runner. The boy was alongside his family while waiting near the finish line.

The explosive devices reportedly contained shrapnel. Local health officials said that a high number of victims had undergone or would require amputation surgery.

Massachusetts General Hospital surgeon Peter Fagenholz told reporters that many would have to undergo repeat operations in the coming days, according to the news agency Reuters.

Twin explosions occurred 50 to 100 yards (45-91 meters) apart around 3 p.m. local time (1900 UTC) near the north side of Bolyston Street. Roughly 90 minutes later, a fire - which police believe was unrelated - erupted at the John F. Kennedy Library in Boston.

The annual Boston marathon attracts over 20,000 runners and about 500,000 spectators annually. A local state holiday commemorating the first battles of the American Revolution, known as Patriots' Day, coincides with the sporting event.

kms/jr (AP, AFP, Reuters, dpa)