BOSTON (AP) - The explosives that killed three and wounded more than 170 at the Boston Marathon were made of pressure cookers packed with metal and ball bearings, a person briefed on the investigation said on Tuesday.

A person briefed on the attack, which left the streets splattered with blood and glass, said the explosives were in six-liter pressure cookers and placed in black duffel bags that were placed on the ground. The person said the duffel bags contained shards of metal, nails and ball bearings.

Mr Richard DesLauriers, FBI agent in charge in Boston, said earlier that investigators had received "voluminous tips" and were interviewing witnesses and were analysing the crime scene.

"We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice," he said.

Massachusetts Governor Deval Patrick said on Tuesday no unexploded bombs were found at the marathon site. He said the only explosives were the ones that went off Monday.

WBZ-TV reported late on Monday that law enforcement officers were searching an apartment in the Boston suburb of Revere. Massachusetts State Police confirmed that a search warrant related to the investigation into the explosions was served Monday night in Revere, but provided no further details.

Some investigators were seen leaving the Revere house early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.

Investigators refused to give any specifics on the bombs and say, for example, where they might have been hidden or whether they were packed with shrapnel for maximum carnage, as is often the case in terror bombings overseas.

But Dr Stephen Epstein of the emergency medicine department at Beth Israel Deaconess Medical Centre, said he saw an X-ray of one victim's leg that had "what appears to be small, uniform, round objects throughout it - similar in the appearance to BBs". He said it remained to be determined what exactly the objects were.