FBI agents zeroed in Tuesday on how the Boston Marathon bombing was carried out - with kitchen pressure cookers packed with explosives, nails and other lethal shrapnel - but said they still didn't know who did it and why.
An intelligence bulletin issued to law enforcement and released late Tuesday includes a picture of a mangled pressure cooker and a torn black bag the Federal Bureau of Investigation says were part of a bomb.
- Egypt, Saudi Arabia condemn Boston bombings
- 2 explosions at Boston marathon; 3 dead
- Obama: Boston bombing 'act of terror'
The FBI and other prominent law enforcement agencies repeatedly pleaded for members of the public to come forward with photos, videos or anything suspicious they might have seen or heard.
President Barack Obama branded the attack an act of terrorism but said officials don't know "whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual."
Remains of bomb (Photo: AP)
Richard DesLauriers, FBI agent in charge in Boston, said at a news conference that the "range of suspects and motives remains wide open." He vowed to "go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime."
Scores of victims of the Boston bombing remained in hospitals, many with grievous injuries, a day after the twin explosions near the marathon's finish line killed three people, wounded more than 170 and reawakened fears of terrorism. A 9-year-old girl and 10-year-old boy were among 17 victims listed in critical condition.
Heightening jitters in Washington, where security already had been tightened after the bombing, a letter addressed to a senator and poisoned with ricin or a similarly toxic substance was intercepted at a mail facility outside the capital, lawmakers said.
Scene in Boston after explosions (Photo: AP)
There was no immediate indication the episode was related to the Boston attack. Senate Majority Leader Harry Reid said the letter was sent to Republican Sen. Roger Wicker of Mississippi.
Officials found that the bombs in Boston consisted of explosives put in ordinary, 1.6-gallon (6-liter) pressure cookers, one with shards of metal and ball bearings, the other with nails, according to a person close to the investigation who spoke on condition of anonymity because the probe was still going on.
Both bombs were stuffed into black duffel bags and left on the ground, the person said.
DesLauriers confirmed that investigators had found pieces of black nylon from a bag or backpack and fragments of BBs and nails, possibly contained in a pressure cooker. He said the items were sent to the FBI laboratory at Quantico, Virginia, for analysis.
The FBI said it is looking at what Boston television station WHDH said are photos sent by a viewer that show the scene right before and after the bombs went off. The photo shows something next to a mailbox that appears to be a bag, but it's unclear what the significance is.
"We're taking a look at hundreds of photos and that's one of them," said Jason Pack, FBI spokesman in Boston.
Investigators said they have not yet determined what was used to set off the explosives.
Pressure-cooker explosives have been used in international terrorism, and have been recommended for lone-wolf operatives by al-Qaeda's branch in Yemen.
But information on how to make the bombs is readily found online, and US officials said Americans should not rush to judgment in linking the attack to overseas terrorists.
DesLauriers said there had been no claim of responsibility for the attack.
The blasts killed 8-year-old Martin Richard of Boston, 29-year-old Krystle Campbell of Medford, Massachusetts, and a third victim, identified only as a graduate student at Boston University.
On Tuesday, an official at the Chinese Consulate in New York, who was not authorized to give his name, confirmed that student was a Chinese national. He declined to provide further details.
The Consulate also said in a statement Tuesday that another Chinese citizen was wounded and was in stable condition following surgery.
Phoenix Satellite Television Holdings, a Hong Kong-based broadcaster with ties to the Chinese government, said the deceased was a woman from the northeastern Chinese city of Shenyang who was a graduate student in statistics at Boston University.
In the wake of the attack, security was stepped up around the White House and across the country.
Police massed at federal buildings and transit centers in Washington, critical response teams deployed in New York City, and security officers with bomb-sniffing dogs spread through Chicago's Union Station.
Homeland Security Secretary Janet Napolitano said that the stepped-up security was a precaution and that there was no evidence the bombings were part of a wider plot.
- Receive Ynetnews updates
directly to your desktop