The search for whoever caused the Boston Marathon bombings is broad, according to the FBI.

Right now it is not known who is behind the bombing, and whether any groups are involved.

The two bombs that ripped through the crowds at the Boston Marathon, killing three people and wounding more than 170, were fashioned out of pressure cookers and packed with shards of metal, nails and ball bearings, a person briefed on the investigation said Tuesday.

The details on the apparently crude but deadly explosives emerged as investigators appealed to the public for amateur video and photos that might yield clues, and the chief FBI agent in Boston vowed "we will go to the ends of the Earth" to find those responsible.

A person who spoke on the condition of anonymity the explosives were put in 6-liter pressure cookers, placed in black duffel bags and left on the ground. They were packed with shrapnel to inflict maximum carnage, the person said.

The person said law enforcement officials have some of the bomb components but do not yet know what was used to set off the explosives.

A doctor treating the wounded appeared to corroborate the person's account, saying one of the victims was maimed by what looked like ball bearings or BBs. Doctors also said they removed a host of sharp objects from the victims, including nails that were sticking out of one little girl's body.

"They can give me a cavity search right now and I'd be perfectly happy," said Daniel Wood, a video producer from New York City who was waiting for a train.

Similar pressure-cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 joint FBI and Homeland Security intelligence report. Also, one of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the intelligence report said.

Two of the those who were fatally injured have been identified. Eight-year-old Martin Richard was standing near the finish line when one of the bombs exploded. Also killed was Krystle Campbell, a 2001 graduate of Medford (Mass.) High School.

Third victim: Chinese citizen, BU grad student

The third victim was identified as a Chinese citizen, according to China's consulate in New York, and a grad student at Boston University. 

In a statement late Tuesday afternoon, the school said it was not releasing the name or any other information about the student, pending permission from the family.

The statement says the student was with two friends who were watching the race at the finish line, not far from the university's campus. One of the friends, also a grad student at the university, was injured and is at Boston Medical Center in stable condition.

Boston's police commissioner Tuesday said there were more police officers at the marathon and the finish line Monday than in previous years because of growing attendance numbers.

He also said that part of the investigation will be to determine whether the bombings could have been prevented -- and why they weren't.

Obama: Explosions being investigated as terrorism

President Barack Obama says the explosions at the Boston Marathon are being investigated as an act of terror, although authorities still don't know who is responsible.

He called the bombing "a heinous and cowardly act" used to target innocent civilians. Obama spoke to reporters at the White House after a briefing by his national security team.

President Obama will be onhand for an interfaith service Thursday morning at the Cathedral of Holy Cross in the south end of Boston.

Meanwhile, police and federal agents appealed to the public Tuesday for amateur video and photos that might yield clues to the Boston Marathon bombing as the chief FBI agent in Boston vowed "we will go to the ends of the Earth" to find whoever carried out the deadly attack.

Two bombs blew up seconds apart Monday at the finish line of one of the world's most storied races, tearing off limbs and leaving the streets spattered with blood and strewn with broken glass. Officials said there were 176 casualties at hospitals, at least 17 of whom were critically injured. At least eight children were being treated at hospitals.

FBI takes the lead in the investigation

The FBI took charge of the investigation, converging on a home in the suburb of Revere on Monday night. Authorities gave no details on the search. Investigators were seen leaving a building there early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.

They also repeatedly appealed for any video, audio and photos taken by marathon spectators, even images that people think might not think are significant.

"There has to be hundreds, if not thousands, of photos and videos" that might help investigators, state police Col. Timothy Alben said.

Boston Police Commissioner Edward Davis said investigators gathered a large number of surveillance tapes from businesses and intend to go through them frame by frame.

Investigators refused to give any specifics on the bombs and say, for example, where they might have been hidden or whether they were packed with shrapnel for maximum carnage, as is often the case in terror bombings overseas.

But Dr. Stephen Epstein of the emergency medicine department at Beth Israel Deaconess Medical Center said he saw an X-ray of one victim's leg that had "what appears to be small, uniform, round objects throughout it - similar in the appearance to BBs." He said it remained to be determined what exactly the objects were.

Bombs went off seconds apart

Boston Police Commissioner Ed Davis said the explosions happened within seconds of each other, at two locations that were 50 to 100 yards apart, and that each explosion resulted in "multiple casualties."

President Barack Obama was careful not to use the words "terror" or "terrorism" as he spoke at the White House Monday after the deadly bombings, but an administration official said the attack was being treated as an act of terrorism.

"We will find out who did this. We'll find out why they did this," the president said. "Any responsible individuals, any responsible groups, will feel the full weight of justice."

Many victims suffered lower leg injuries and shrapnel wounds, which officials say indicates the device was low to the ground. Some suffered ruptured eardrums.

Mass General Trauma Surgeon Peter Fagenholz described the injuries as combined injuries, meaning the bone, soft tissue and vascular systems are injured.

He said medical staff members are pulling small bits of metal out of people in the emergency room, which analysts say suggests the bombs were designed to propel shrapnel.

Fagenholz said many of the victims will need to undergo repeat surgical procedures on Tuesday.

Boston doctor: "This is what we expect from war"

The FBI took charge of the investigation into the bombings, serving a warrant late Monday on a home in suburban Boston and appealing for any video, audio and still images taken by marathon spectators.

The fiery explosions took place about 10 seconds and about 100 yards apart, knocking spectators and at least one runner off their feet, shattering windows and sending dense plumes of smoke rising over the street and through the fluttering national flags lining the route.

Blood stained the pavement, and huge shards were missing from window panes as high as three stories. Victims suffered broken bones, shrapnel wounds and ruptured eardrums.

Roupen Bastajian, a state trooper from Smithfield, R.I., had just finished the race when he heard the explosions.

"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated. ... At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."

At Massachusetts General Hospital, Alasdair Conn, chief of emergency services, said: "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."

President: Suspect will feel ''full weight of justice''

President Barack Obama spoke about the marathon bombings later on Monday. He said that investigators do not have "all the answers" in the bombings.

"We still do not know who did this or why, and people shouldn't jump to conclusions before we have the facts," he said. "But make no mistake: we will get to the bottom of this. We will find out who did this. Any responsible individuals, any responsible groups will feel the full weight of justice."

Obama expressed confidence in the people of Boston in the aftermath of the bombings.

"Boston is a tough and resilient town," he said. "So are its people. I am supremely confident that Bostonians will pull together, take care of each other, and move forward as one proud city and as they do, Americans will be with them every single step of the way."