"Anytime bombs are used to target innocent civilians, it is an act of terror," President Obama said Tuesday, addressing the nation for a second time after the bombings that killed three people and injured at least 178. "What we don't yet know, however, is who carried out this attack, or why."
Obama will head to Boston on Thursday for an interfaith memorial service at the Cathedral of the Holy Cross, Massachusetts Gov. Deval Patrick's office announced. That will give the president yet another occasion to claim the mantle of America's mourner-in-chief, the grim task that he has taken on after mass shootings at Fort Hood, in Tucson and most recently in Newtown, Conn.
Memories of that elementary-school massacre were rekindled Tuesday as a picture circulated on the Internet of 8-year-old Martin Richard, the third-grader from Dorchester, Mass., who was killed by the blast as he stood next to his seriously injured mom and sister, all there to watch Martin's dad run. The photo showed Martin holding a sign festooned with hearts and a peace sign in his classroom that read, "No more hurting people. Peace."
Authorities released the name of another of the three fatalities, Krystle Campbell, 29, a restaurant worker from Medford, Mass., who had attended the marathon with her friend to photograph the friend's boyfriend as he ran past. William Campbell says his daughter, who worked at a restaurant in nearby Arlington, was a "very caring, very loving person, and was daddy's little girl." He says the loss has devastated the family.
The third fatal victim was identified Tuesday night as a Chinese national who was a Boston University graduate student, but no name was released.
The official injury toll continued to rise throughout the day. Authorities said a surprisingly large number of the more serious victims lost limbs in the blast, probably because the explosive devices were planted at ground level.
One victim who received considerable attention when his rescue in a wheelchair was captured in an already iconic Associated Press photo - Jeff Bauman Jr., 27, there to watch his girlfriend run - had both of his legs amputated, according to his father. Bauman lives in Chelmsford, Mass., but reportedly lived in the Philadelphia area for a number of years and roots for the Flyers.
He was one of dozens of people with Philadelphia ties who were near the marathon finish line when the twin blasts went off, roughly 12 seconds apart, at 2:50 p.m. Monday, as a throng of the more than 23,000 runners near the back of the pack were completing the 26.2-mile race.
One area man with a particularly dramatic story is Dr. Howard Palamarchuk, director of sports medicine at Temple University's School of Podiatric Medicine. He's been volunteering at the Boston Marathon for 28 years and was there Monday with nine of his students. The Philadelphians were expecting to treat runners with twisted ankles, not bomb victims with severed limbs.
"They weren't runners in shorts and T-shirts anymore," Palamarchuk said. "They were grandmothers, young ladies with their husbands, grandfathers, and parents. You're not used to seeing people come in there with really horrendous wounds." He added that "I looked around and thought this is what 9/11 must have looked like."
There was one big difference from the 2001 attacks, however. In that case, the FBI identified the terror group al Qaeda as the perpetrators in a matter of hours. But the Boston attack has instead transitioned into a slow-moving police procedural, as the FBI-led task force sifted through about 2,000 tips from the public, numerous videos and photos of the crime scene, and recovered pieces of black canvass that likely came from the bags that held the bombs.
Government and law-enforcement officials began to offer some clues into how the bombs were constructed. House Homeland Security Chairman Michael McCaul, a Texas Republican, told the website Talking Points Memo on Tuesday night that the devices apparently contained gunpowder. The Boston Globe reported the bombs "consisted of two 6-liter pressure cookers packed with nails, ball bearings, and other metal" that were placed in black duffel bags and detonated with circuit boards.
Heightening jitters in Washington, where security already had been tightened after the bombing, a letter poisoned with ricin or a similarly toxic substance was intercepted at a mail facility. There was no immediate indication the letter, addressed to Sen. Roger Wicker, of Mississippi, was related to the Boston attack.
The painstakingly slow pace of the probe did not prevent an outpouring of solidarity for the victims from citizens in Boston and around the country. In New York's Yankee Stadium, thousands of fans sang the theme song of their archrival Boston Red Sox: Neil Diamond's "Sweet Caroline."
Several hundred people turned out Tuesday evening for a candlelight vigil on the Boston Commons with banners declaring "Peace here and everywhere" and the signature lyric of the 1966 hit "Dirty Water" by the Standells: "Boston, you're our home."
The Associated Press and Philly.com's Frank Kummer contributed to this article.
On Twitter: @Will_Bunch