BOSTON — 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 today.

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 said that the explosives were put in 6-liter pressure cookers, placed in black duffel bags and left on the ground. They were packed with shrapnel, 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.

Earlier, 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.

At the White House, meanwhile, President Barack Obama said that the bombings were an act of terrorism but that investigators do not know if they were carried out by an international organization, domestic group or a “malevolent individual.”

He added: “The American people refuse to be terrorized.”