BOSTON (Reuters) - Investigators of the Boston Marathon bombings believe they have identified a suspect from security video taken before Monday's blasts killed three people and injured 176 others, a U.S. law enforcement source said on Wednesday.
The source said an official announcement was expected later on Wednesday in what is the first major publicly disclosed break in the investigation.
Investigators of the bombings searched thousands of pieces of evidence from cell phone pictures to shrapnel shards pulled from victims' legs.
Based on shards of metal, fabric, wires and a battery recovered at the scene, the focus turned to whoever may have made bombs in pressure cooker pots and taken them in heavy black nylon bags to the finish line of the world-famous race watched by crowds of spectators.
A stretch of Boston's Boylston Street almost a mile long and blocks around it remained closed as investigators searched for clues in the worst attack on U.S. soil since the hijacked plane strikes of September 11, 2001.
Cities across the United States were on edge after Monday's blasts in Boston. Adding to the nervousness was the announcement that mail containing a suspicious substance addressed to a lawmaker and to President Barack Obama. The FBI said, however, that agents had found no link the attack in Boston.
The blasts at the finish line of Monday's race injured 176 people and killed three: an 8-year old boy, Martin Richard, a 29-year-old woman, Krystle Campbell and a Boston University graduate student who was a Chinese citizen.
Boston University identified the student as Lu Lingzi.
No one has claimed responsibility for the attack.
"Whether it's homegrown, or foreign, we just don't know yet. And so I'm not going to contribute to any speculation on that," said U.S. Secretary of State John Kerry, who until January was Massachusetts' senior senator. "It's just hard to believe that a Patriots' Day holiday, which is normally such time of festivities, turned into bloody mayhem."
FBI ASKS WITNESSES FOR PHOTOS
The FBI was leading the investigation and asking witnesses to submit any photos of the blast site -- which was crowded with tens of thousands of spectators, race staff and volunteers and runners. Many of them have turned in thousands of images, authorities said.
"Probably one of the best ways to get a lead is to go through those images and track down people coming and going with backpacks," said Randy Law, an associate professor of history at Birmingham-Southern College in Alabama and author of "Terrorism: A History."
"It's the needle in the haystack but when you have the resources that the local and federal authorities have, they can go through what I'm sure will be thousands and thousands of photos and hours of videos. You can find something occasionally," Law said.
The head of trauma surgery at Boston Medical Center, which was still treating 19 victims on Wednesday, said his hospital was collecting the shards of metal, plastic, wood and concrete they had pulled from the injured to save for law enforcement inspectors. Other hospitals were doing the same.
"We've taken on large quantities of pieces," Dr. Peter Burke of Boston Medical Center told reporters "We send them to the pathologists and they are available to the police."
NYLON FRAGMENTS, BALL BEARINGS AND NAILS
Bomb scene pictures produced by the Boston Joint Terrorism Task Force and released on Tuesday show the remains of an explosive device including twisted pieces of a metal container, wires, a battery and what appears to be a small circuit board.
One picture shows a few inches of charred wire attached to a small box, and another depicts a half-inch (1.3 cm) nail and a zipper head stained with blood. Another shows a Tenergy-brand battery attached to black and red wires through a broken plastic cap. Several photos show a twisted metal lid with bolts.
The nickel metal hydride battery typically is used by remote-controlled car enthusiasts, said Benjamin Mull, a vice president at Tenergy Corp. The batteries, made in Shenzhen, China, are sold on the internet and in hundreds of outlets.
People at the company "were shocked and appalled" when they learned their battery had been used in the blast, he said.
Security officials who spoke on condition of anonymity said instructions for building pressure-cooker bombs similar to the ones used in Boston can be found on the Internet and are relatively primitive.
Pressure cookers had also been discovered in numerous foiled attack plots in both the U.S. and overseas in recent years, including the failed Times Square bombing attempt on May 1, 2010, the officials said.
(Additional reporting by Mark Hosenball in Washington, Svea Herbst-Bayliss in Boston and Terril Yue Jones in Beijing; Editing by Daniel Trotta and Grant McCool)