BOSTON -- Federal 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.

The FBI and other law enforcement agencies repeatedly appealed to the public to come forward with photos, videos or anything suspicious they might have seen or heard.

"The range of suspects and motives remains wide open," Richard DesLauriers, FBI agent in charge in Boston, said at a news conference. He vowed to "go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime."

Martin Richard was watching the Boston Marathon with his parents, brother and sister in the area of the finish line when he was killed. Courtesy Richard Family

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."

He added: "The American people refuse to be terrorized."

The bombs exploded 10 or more seconds apart, tearing off victims' limbs and spattering streets with blood, instantly turning the festive race into a hellish scene of confusion, horror and heroics.

The blasts killed 8-year-old Martin Richard of Boston, 29-year-old Krystle Campbell of Medford, Mass., and Lu Lingzi, a graduate student at Boston University.

Richard's mother, Denise, and 6-year-old sister, Jane, were badly injured. His father, Bill, and brother also were watching the race but were not hurt.

"My dear son Martin has died from injuries sustained in the attack on Boston. My wife and daughter are both recovering from serious injuries. We thank our family and friends, those we know and those we have never met, for their thoughts and prayers," Bill Richard said in a statement released Tuesday. "I ask that you continue to pray for my family as we remember Martin. We also ask for your patience and for privacy as we work to simultaneously grieve and recover. Thank you."

A candle burned on the stoop of the family's single-family home in the city's Dorchester section Tuesday, and the word "Peace" was written in chalk on the front walk.

Neighbor Betty Delorey said Martin loved to climb neighborhood trees and hop the fence outside his home.

Campbell's father, William, said Tuesday that Krystle had gone with her best friend to take a picture of the friend's boyfriend crossing the finish line on Monday afternoon.