(CBS/AP) BOSTON - FBI pictures obtained by the Associated Press late Tuesday depict parts of the pressure cookers believed to have held the bombs that erupted near the finish line of the Boston Marathon Monday,reports.
An intelligence bulletin issued to law enforcement and obtained by the AP included pictures of a mangled pressure cooker, a torn black bag, a circuit board and a battery connected to wires, all of which the bulletin said were from the two bombs used in Monday's attack.
Both explosive devices appear to have been placed in metal pressure cookers packed with nails and ball bearings designed to amplify the damage from the explosions, CBS News correspondent Bob Orr reports.
Investigators believe the bombs were hidden in black nylon backpacks and housed inside the pressure cookers, which can help boost the power of relatively small devices by briefly constraining the blast. Upon explosion, the pressure cooker bombs can add large chunks of metal to the shrapnel spray, according to CBS News.
Pressure cooker explosives have been used in international terrorism, and have been recommended for lone-wolf operatives by Al-Qaida's branch in Yemen. But information on how to make the bombs is readily found online, and U.S. officials said Americans should not rush to judgment in linking the attack to overseas terrorists.
The FBI and other law enforcement agencies repeatedly pleaded for members of the public to come forward with photos, videos or anything suspicious they may have seen or heard.
"The range of suspects and motives remains wide open," Richard DesLauriers, the FBI agent in charge in Boston, said at a news conference Tuesday. He vowed to "go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime."
DesLauriers confirmed that pieces of black nylon from a bag or backpack and fragments of BBs and nails possibly contained in a pressure cooker were sent to the FBI laboratory at Quantico, Va., for analysis.
Law enforcement sources told CBS News a Saudi Arabian man who was being questioned by investigators is not considered a suspect at this time, and it appears he was a spectator who was injured in the attack.
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."
Obama plans to attend an interfaith service Thursday in the victims' honor in Boston. He has traveled four times to cities reeling from mass violence, most recently in December after the schoolhouse shooting in Newtown, Conn.
The bombs exploded 10 or more seconds apart Monday near the marathon's finish line, killing 8-year-old Martin Richard and 29-year-old Krystle Campbell. The Shenyang Evening News, a state-run Chinese newspaper, identified the third victim as Lu Lingzi, a graduate student at Boston University.
More than 170 people were wounded and scores of victims remained in hospitals Tuesday, many with grievous injuries. A 9-year-old girl and 10-year-old boy were among 17 victims listed in critical condition.
"We've removed BBs, and we've removed nails from kids. One of the sickest things for me was just to see nails sticking out of a little girl's body," said Dr. David Mooney, director of the trauma center at Boston Children's Hospital.
At Massachusetts General Hospital, all four amputations performed there were above the knee, with no hope of saving more of the legs, said Dr. George Velmahos, chief of trauma surgery.
"It wasn't a hard decision to make," he said. "We just completed the ugly job that the bomb did."
DesLauriers said there was no claim of responsibility for the attack.
He urged people to come forward with anything suspicious, such as seeing someone carrying a dark heavy bag at the race or hearing someone express interest in explosives or desire to attack the marathon.
"Someone knows who did this," DesLauriers said. "Importantly, the person who did this is someone's friend, neighbor, co-worker or relative."
Investigators in the Boston bombing also are combing surveillance tapes from businesses around the finish line and asking travelers at Logan Airport to share any photos or video that might help.
"This is probably one of the most photographed areas in the country yesterday," said Boston police Commissioner Edward Davis. He said two security sweeps of the marathon route had been conducted before the bombing.
Boston police and firefighter unions announced a $50,000 reward for information leading to arrests.