The two powerful blasts that killed at least three people, including an eight-year-old, and wounded about 170 near the finish line of the Boston Marathon at Back Bay section on Monday were made from ordinary kitchen pressure cookers, officials said.
"The pressure cookers were filled with nails, ball bearings and black powder, and the devices were triggered by "kitchen-type" egg timers," The New York Times reported quoting an official.
The bombs were reportedly enabled enough to send the shrapnel tear through the skin and muscle, the report said.
Some of the metals even destroyed the lower limbs of some victims who had only bits of tissue left that held parts of their legs together when they arrived at the Massachusetts General Hospital, reports said.
According to Boston police, at about 2:50 pm, on Monday (US time) two large explosions along the Boston Marathon route in the area of 673 Boylston Street shook the area which turned into a scene of blood and utter chaos.
United States President Barack Obama had earlier confirmed the attack as an act or terror and vowed justice.
While the FBI is probing the attack, the White House had said in a statement: "...., we don't yet know who carried out this attack, and a thorough investigation will have to determine whether it was planned and carried out by a terrorist group, foreign or domestic."
The first bomb was apparently placed in a garbage and the explosion took place around 2:50 p.m., which was more than four hours after the race had started.
The second bomb had exploded within seconds of the first one and the explosion took place several hundred yards away from the first spot.
As the reverberation of the blast was felt even outside the city, the security level was reportedly heightened on public transit, with shutting down streets near the White House, including Pennsylvania Avenue.
President Barack Obama, condemning the attack, referred to the Patriots´ Day holiday and called it "a day that celebrates the free and fiercely independent spirit that this great American city of Boston has reflected from the earliest days of our nation."
"We will get to the bottom of this. We will find who did this, and we will find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice," Obama had said.
According to Federal Bureau of Investigation (FBI) of United States, a multi-agency response including state and federal law enforcement agencies has been activated and is investigating the cause of the explosions along the Boston Marathon route and elsewhere.