Boston Marathon Bomber Loose as Authorities Race for Clues

April 16, 2013 Updated Apr 16, 2013 at 8:28 AM EDT

By Brian Ross, ABC News

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April 16, 2013 Updated Apr 16, 2013 at 8:28 AM EDT

BOSTON, MASS. (ABC News) - Hundreds of U.S. law enforcement agents are racing to gather and analyze every shred of evidence from Monday's deadly Boston Marathon bombing for a hint of the attacker, but the morning after three were killed and nearly 150 more injured in twin explosions, no one has been taken into custody.

Overnight a tip about possible explosives led federal agents to search an apartment on the fifth floor of a building in the Boston suburb of Revere. Agents later told residents there was nothing to worry about, but the quick and overwhelming law enforcement response underscored the urgency of the FBI's effort to track and stop the people responsible.

"This cowardly act will not be taken in stride," Boston Police Commissioner Ed Davis said. "We will turn [over] every rock to find the people responsible for this."

FBI agents also went to a local hospital to question a 20-year-old Saudi college student who was injured in the blast, but authorities stressed that he is not considered a suspect.

Experts said the most important clues could come from several videos from the race that caught the blasts in real time as well as the explosions themselves.

"They want to get the component parts to identify how these bombs were made," former ATF chief Michael Sullivan told ABC News. "It will be extremely useful to identify how these things were made and how they were activated, detonated."

Authorities said the white smoke seen shortly after one of the detonations indicates small bombs with a simple, low-velocity, explosive mixture -- not military grade.

"[The attackers] may not have the resources as we have seen in other bomb attacks, but they knew how to make the bomb go BOOM," said Nick Casale, security expert and former NYPD officer.

Experts also pointed to large pieces of medal, seen flying through the air in one of the videos, which suggests the bomb may have been concealed in a mail box or trash barrel. The limited damage to nearby buildings tells experts the bomb may have failed to fully function as designed.

If it did somehow malfunction, former NYPD bomb squad member Kevin Barry said Boston was potentially saved from "much more catastrophic injury and possibly more death."

Richard Clarke, a former counter-terrorism advisor to the White House and now ABC News consultant, said the National Security Agency would likely be going back to check on calls going overseas around the time of the attack, as well as any calls made to cell phones at the exact time of the explosions, as some devices can be affixed with the phones for remote detonation.

Today dozens are recovering in Boston hospitals, 17 in critical condition, hours after President Obama told the nation, "Make no mistake, we will get to the bottom of this and we will find out who did this."

"Any individuals, any responsible groups will feel the full weight of justice," Obama said.

ABC News' Angela M. Hill, Cindy Galli, Matthew Mosk and Lee Ferran contributed to this report.