Twin blasts at Boston Marathon kill 3, injure over 144

Terrorism struck the US hard on Monday, when twin explosions near Boston Marathon finish line, killed three people and injured over 144.

The sporting event – considered the world's premier marathon, which began in 1897 - turned into a tragic bloody spectacle with dismembered bodies strewn over blood stained pavement and shattered glass pieces.

TV visuals showed panic-struck people rushing, and dense plumes of smoke rising over the streets.

An eight year old boy is among the three dead, and out of 144 injured, at least 17 are said to be in a critical state.

Most of the injured have suffered dismemberment in the fiery twin blasts. A horrific chorus of high-pitched wailing and screaming rang out as bewildered runners and spectators fled the carnage and debris.

The thunderous blasts struck near the finish line of the marathon, long after the winners had crossed. Competitors who were still running when the blasts rocked downtown Boston were diverted elsewhere. Some 27,000 people were entered to take part in the event.

Video footage on American TV showed the moment when the first blast apparently struck: the detonation came on the left side of the course, behind spectators and a row of colourful national flags showing how runners come from around the world to take part.

Security personnel in yellow jackets threw their hands to their ears as the blast took place and at least one runner was thrown to the ground as white smoke billowed upward. The already waving flags whipped violently with the shockwave of the explosion.

The investigation has been taken over by the Federal Bureau of Investigation, which is treating the act as a terror strike.

The probe into Boston bombings has got some active leads with good progress in forensic investigation, a law enforcement official said.

The FBI has been examining surveillance cameras in the area and issuing subpoenas for records from cell towers in the area to isolate and trace calls from around Copley Square at the time of the blasts, according to the official.

Investigators have warned police officers to be on the lookout for a "darker-skinned or black male" with a possible foreign accent in connection with the Marathon bombing, CNN said citing a law enforcement advisory.

The man was seen with a black backpack and sweatshirt and was trying to get into a restricted area about five minutes before the first explosion, the lookout notice states.

President Barack Obama went on national television to say it was not yet clear who was behind the blasts. He said the perpetrators would pay. He did not utter the word "terror."

"We still do not know who did this or why. And people shouldn't jump to conclusions before we have all the facts," Obama said. "But make no mistake, we will get to the bottom of this, and we will find out who did this, we'll find out why they did this."

A senior White House official, speaking on condition of anonymity, said later that "any event with multiple explosive devices - as this appears to be - is clearly an act of terror."

Grisly accounts of the blasts abounded. "We saw people with their legs blown off," Mark Hagopian, owner of the Charlesmark Hotel, told AFP from the basement of a restaurant where he had sought shelter.

"A person next to me had his legs blown off at the knee - he was still alive."

"It was bad, it was fast," he said. "There was a gigantic explosion ... we felt wind on our faces... Police were saying: 'Get out, get out, leave, leave there may be more bombs.'"

Cities from New York to Los Angeles went on high alert following the blasts.

The New York Police Department stepped up security at hotels and landmark locations like the World Trade Center site, Empire State Building and Times Square following the explosions.

While officials said there was no report of a specific threat to New York, Mayor Michael Bloomberg urged city residents to be vigilant as police took both highly visible and unseen precautions.

Security was also stepped up in Washington – another site of the 9/11 attacks - as well as in Los Angeles and San Francisco.