Trace Boston Marathon Pressure Cooker Bomb

The bombs that ripped through the crowd at the Boston Marathon were fashioned out of pressure cookers and packed with metal shards, nails and ball bearings to inflict maximum carnage, a person briefed on the investigation said Tuesday.

Doctors say they removed a host of sharp objects from children and adults injured by the Boston Marathon explosions.

"We've removed BBs and we've removed nails from kids. Two children remain in critical condition at the hospital with serious leg injuries. Doctors at Massachusetts General Hospital also say they removed metal fragments from victims of the two bombs. Massachusetts General treated 31 victims of the bombs.

Residents have described the terrifying scenes in Watertown at a shootout between the two Boston marathon terror suspects and police, which left one officer dead.

It is believed the two suspects held a hostage for half an hour while police pursued the car to Watertown, where explosives were thrown from the car at police and gunfire was exchanged, a statement said.

Gunshot, gunshot, gunshot  gunshot. 'About 70 or 80 yards down the street was six police vehicles. There was a long exchange of gunfire and shooters had hand guns.'

It covered the street with smoke.

Peter Jennings, 33, said he woke to the sound of an explosion just before 1am at home on Prentiss Street in Watertown.

I looked out the window, and it was like nothing I’ve ever seen – blue light after blue light after blue light.’

There were about 10 cop cars, they took a left on Mount Auburn Street heading toward Galen Street.’

Residents of Watertown and surrounding neihbourhoods of Waltham, Newton, Belmont, Cambridge and Allston Brighton have been instructed by the Boston Police Department to stay at home and to not answer their doors unless instructed by a police officer.

Bombers who killed three people at the Boston Marathon Monday used a pressure cooker pot that's widely available for as little as $140 from major retail stores to house at least one of the explosives.

Investigators said Tuesday they had recovered a mangled Fagor brand pressure cooker pot, shrapnel, a circuit board and wiring from what they said was a partially exploded device near the finish line. 

Two bombs exploded Monday, but investigators said there was not enough evidence to determine if the second bomb was also built from a pressure cooker.

DesLauriers said then investigators have received "voluminous" tips and brought in additional evidence response teams to help deal with the massive workload.