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(NBC NEWS) -- Federal investigators continue to scour several city blocks surrounding the scene of Monday's attack on the Boston Marathon.

They're looking for clues which might help them discover who detonated two bombs near the finish line.

Investigators believe people who were near the marathon finish line in the hours and seconds before the bombing hold the key to solving this crime and getting justice for those who were killed and injured.

The FBI says it's gotten more than 2,000 tips, pictures and videos from the scene of Monday's fatal bombing.

Investigators say the explosive devices contained ball bearings, BBs and nails to increase the damage inflicted on victims.

"Both explosives were placed in black nylon backpack," said the FBI's Richard DesLauriers. "It would have been heavy because of the components believed to be in it."

The ATF is searching a 12 block area around Copley Square.

No piece of evidence is considered too small to help.

"They are literally going over the crime scene with a fine tooth comb, and there is a lot of information coming from the evidence and from our analysis of video," said Boston Police Commissioner Edward Davis.

There are still no known suspects.

"This is a guy who wanted to kill a hundred people, and that is significant to the investigators to try and have that insight who there are dealing with," says retired ATF Special Agent James Cavanaugh.

Secretary of Homeland Security Janet Napolitano testified before Congress Wednesday, stating that there is still no evidence the attack was part of any larger plot.

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