1. Latest Figures
In addition to the three people who died as result of the blast, 170 others were injured. As of Wednesday, 76 patients remain in Boston hospitals, with 20 in critical condition (including a 9-year-old girl and 10-year-old boy).
Massachusetts General Hospital chief of trauma surgery Dr. George Velmahos said four patients had to undergo above-the-knee amputations.
"It wasn't a hard decision to make," Velmahos said. "We just completed the ugly job that the bomb did."
2. Victims Identified
Martin Richard: The first of three victims identified, the 8-year-old Boston boy was a third-grader at the Neighborhood House Charter School in Dorchester, Mass., where he enjoyed activities such as soccer, basketball and baseball. He was just getting back from getting ice cream prior to the explosion. Martin's mother, Denise, and sister Jane, 6, were also hurt in the attack. (Denise suffered serious a brain injury; Jane lost a leg.)
Krystle Campbell: The 29-year-old Medford, Mass. native, a restaurant manager, was at the race to support her best friend's runner boyfriend. She was at the finish line with her pal trying to get into position for when he crossed, but the bombs detonated beforehand. Krystle's father William Campbell described her as "just a very caring, very loving person, and was daddy's little girl."
Lu Lingzi: The Shenyang Evening News, a state-run Chinese newspaper, identified Lingzi as the third victim in the blast. A graduate student (in statistics) at Boston University, Lingzi was at the race with two friends, one of whom was also hurt when the bombs blew.
3. Search for the Suspects
No individual or group has come forward taking credit for the attack as of Wednesday. The FBI continues to ask the public for any audio, video or photos racegoers took on Monday. Boston bureau head Richard DesLauriers appealed to the public to dig deep for any clues connected to the incident.
"Someone knows who did this; importantly, the person who did this is someone's friend, neighbor, co-worker or relative," DesLauriers said, adding that more than 2,000 tips had been submitted to authorities.
Police and fire departments in the Boston area have offered a $50,000 reward for information leading to arrests of those responsible.
4. Pressure Cooker Bombs
Officials said the bombs involved in the attack incorporated the use of 1.6-gallon kitchen pressure cookers filled with elements meant to injure and maim -- including metal shards, ball bearings, nails, explosives and shrapnel.
The bombs were transported to the race in a black nylon bag, according to an intelligence bulletin police received. The remnants of the explosive device have been forwarded to the FBI's labs in Quantico, Va. The specific use of pressure cookers might link the bombers to Afghanistan, India, Nepal and Pakistan, as all areas were cited as hotbeds for use of the devices in a July 2010 report from the Homeland Security Department and the FBI.
5. Symbol of Senseless Damage
The man in the widely-circulated AP photo who lost both of his legs in the blasts has been identified as 27-year-old Jeff Bauman Jr., who was at the event to cheer on his runner girlfriend. His legs were amputated at Boston Medical Center due to vascular and bone damage, his father Jeff Bauman said in a Facebook post.
The father wrote: "Unfortunately my son was just in the wrong place at the wrong time. I just can't explain what's wrong with people today to do this to people. I'm really starting to lose faith in our country."
6. Country on Alert
High population areas such as New York City and Chicago's Union Station were under heavy security, incorporating the use of bomb-sniffing dogs, with critical response teams on standby. In Washington D.C., added security was present at the White House, federal buildings and transit hubs. Homeland Security Secretary Janet Napolitano said the added personnel was in response to Monday's bombings, and that the is no evidence the marathon massacre was part of a larger operation.
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7. Related Incident?
A letter addressed to Mississippi Sen. Roger Wicker tested positive for traces of ricin, a highly-toxic substance. The letter never made it to Washington D.C., as it was tagged and tested at a mail facility on the outskirts of the city. Officials said there was no strong indication the letter was connected to the bombing.