Obama says that those responsible will 'feel the full weight of justice'

Balls of newspaper flew around in the wind on Boston's Boylston Street on Tuesday amid angry mourning for the three dead and more than 170 injured from the twin bomb attack on the city's famed marathon.

President Barack Obama said the explosions at the Boston Marathon are being investigated as an act of terror, although authorities still do not know who is responsible.

He called the bombing "a heinous and cowardly act" used to target innocent civilians.

The finish line of the world's oldest international marathon remained sealed off as police imposed tight security along the central street and around the city, closing one metro station and carrying out bag checks on trains and buses.

Police hunted for clues on Boylston Street, which was still strewn with debris from the two bombs, in the cold spring wind blowing through the city.

Some of the 27,000 race participants went as close as they could to the finish line to pay tribute. "I am leaving today, but I had to come," said Lea Elliasson, 55, who came from Sweden to take part in the tragic 117th Boston Marathon.

Boston city authorities said that counseling will be available at schools and for marathon runners who felt they needed help after the blasts.

Boston mourned in particular the family of Martin Richard, an 8-year-old who was killed in the blast.

The boy's 6-year-old sister lost a leg in the attack and his mother was also gravely injured, media quoted relatives as saying.

8-year-old mourned

A candle was left on the steps of the family home in the Dorchester neighborhood, and the word "peace" was scrawled in chalk on the sidewalk.

Other tragic stories emerged as doctors told how many of the wounded suffered from metal tacks and ball bearings in the bombs. Liz Norden, a mother of five, told the Boston Globe how two of her sons had lost a leg in the blast. Both had gone to Boylston Street to see a friend finish the race.

"Ma, I'm hurt real bad," Norden quoted one of her sons as telling her in an agonizing phone call from the ambulance taking the brothers to a hospital.

Doctors said that many victims had legs blown off by the blast, and some had to have amputations at the scene.

'An act of terror'

Police said the heightened security would last for several days. Many tourist landmarks were closed, and sporting events such as the Boston Celtics basketball game against the Indiana Pacers on Tuesday was canceled because of the tragedy.

A large area of downtown Boston remained cordoned off by police on Tuesday as authorities pursued an investigation into the bombings.

A stretch of Boylston Street and the blocks around it were closed to traffic as police searched for evidence of the identity of who placed the bombs, which were packed with ball bearings to maximize casualties.

The White House said the bombings would be treated as "an act of terror", and Obama vowed that those responsible would "feel the full weight of justice".

AP-Reuters-China Daily