(CNN) -- After explosives killed three people and wounded about 180 others during the Boston Marathon, details continue to trickle in as investigators sort through evidence.
-- Obama declared an emergency in Massachusetts and ordered federal aid to supplement state and local response efforts.
Details on the bombs
-- Investigators say the dual bombs, which exploded 12 seconds apart Monday, were designed to deliver vicious suffering.
-- One was housed in a pressure cooker hidden inside a backpack, the FBI said. The device also had fragments that may have included nails, BBs and ball bearings, the agency said.
-- The second bomb was in a metal container, but it was unclear whether it was in a pressure cooker as well, the FBI said.
-- Photos obtained by CNN show the remains of a pressure cooker found at the scene, along with a shredded black backpack and what appear to be metal pellets or ball bearings.
-- They were sent to the FBI's national laboratory in Virginia, where technicians will try to reconstruct the devices.
-- In the past, the U.S. government has warned federal agencies that terrorists could turn pressure cookers into bombs by packing them with explosives and shrapnel, and detonating them with blasting caps.
-- Authorities sifted through thousands of pieces of evidence and a mass of digital photos and video clips. They have pleaded for the public's help in providing additional leads and images.
The blasts left three people dead.
-- Martin Richard, an 8-year-old boy with a gap-tooth grin and bright eyes. He loved to run and play in his yard.
-- Krystle Campbell, a 29-year-old freckle-faced woman described as having "a heart of gold."
-- The third victim was Lingzu Lu, a graduate student at Boston University, who had moved to the city last fall, making friends and soaking up new experiences.
-- Of the approximately 178 people injured, 66 remain in area hospitals, according to the latest CNN tally. Thirteen of those are still in critical condition, hospital officials said.
- Martin's mother and sister are among the wounded. His mother underwent surgery for a brain injury, and his 6-year-old sister lost her leg.
-- Doctors say dozens of victims suffered injuries to their legs that involve blood vessels, bone and tissue.
-- The bombings resulted in at least 13 amputations and left doctors picking ball bearings out of victims in the emergency room, a terrorism expert briefed on the investigation said.
-- Dr. George Velmahos, head of trauma care at Massachusetts General Hospital, said his team found "numerous" metal pellets and nails inside patients' bodies. "There are people who have 10, 20, 30, 40 of them in their body, or more," Velmahos said.
-- Dr. Ron Walls also said one patient had more than 12 carpenter-type nails. "There is no question some of these objects were implanted in the device for the purpose of being exploded forward," he said.
Terror group links
-- While the clues moved the investigation forward, it is still unclear whether the attack was an act of domestic or foreign terrorism.
-- No connection has been made to any terrorist group or individual.
-- President Barack Obama described the bombings as an act of terrorism, but said it is unclear whether they were the work of a group or "a malevolent individual."
-- The Pakistani Taliban has said it was not involved in the attack.
-- Authorities don't have a sense of what the motive is, an official said.
-- The FBI asked the public to report anyone who talked about targeting the marathon or showed interest in explosives. The agency urged anyone who might have heard explosions in remote areas -- possibly by someone testing a bomb -- or seen someone carrying "an unusually heavy, dark-colored bag" around the time of the attack to come forward.
Three days later, authorities have yet to make an arrest, but have made "significant progress," a law enforcement official said.
-- Investigators have pinpointed two men of "high interest" as possible suspects after they appeared in images near the finish line moments before the blasts, a law enforcement official said.
-- One of the men is seen carrying a black backpack.
-- Authorities had not yet named the two, but their photographs have been distributed to law enforcement, a source said.
-- Earlier Wednesday, two sources with knowledge of the investigation identified a man as a possible suspect in the attack, but did not name him.
-- Seen on a video, the man wore a white baseball cap. One of the sources added that the cap was on backwards and the man was also wearing a light-colored hooded sweatshirt and a black jacket.
-- It was not immediately clear if he is one of those alluded to in the photographs distributed to law enforcement officials.
-- Investigators are closer to cracking the case "every hour," Massachusetts Gov. Deval Patrick said. He urged patience with the probe to allow investigators space to do their job.
-- "I wish they had nailed the perpetrator within minutes of this catastrophe, but I understand from experience it's going to take some time," he said.
-- Significant progress has been made in the investigation, but there has been no arrest, the FBI told CNN.
-- After sources told CNN on Wednesday that an arrest had been made, sources with the Department of Justice and Boston Police Department each said that no arrest had been made.
-- A source with Boston law enforcement said, "We got him," but would not clarify whether that meant an identification had been made or an arrest carried out.
-- Some federal sources said that it would be going too far to say even that an identification had been made, but several sources in Boston disputed that, saying a clear identification had been made.
-- Investigators believe they have identified a suspect in the marathon bombings, a source who has been briefed on the investigation told CNN's John King. The possible breakthrough came from an analysis of video from a Lord and Taylor department store near the site of the second blast, and video from a Boston TV station helped as well, the official said.
-- As they scramble to search for suspects, authorities are processing "the most complex crime scene that we have dealt with in the history of our department," said Ed Davis, the Boston police commissioner.
-- Forensic specialists and dogs trained to detect explosive devices and their residue were at the scene of the blasts. A command post has been created, with 1,000 officers conducting interviews and gathering details.
-- Authorities have received 2,000 tips from around the world, said Richard DesLauriers, the special agent in charge of the FBI's Boston office. "Someone knows who did this," he said.
-- Cities have stepped up security after the attacks, including Washington, New York, Miami, Chicago, Atlanta and Los Angeles.
-- British police said they are reviewing the security plan for the London Marathon scheduled for Sunday. The marathon will observe 30 seconds of silence and is urging runners to wear a black ribbon to mark the Boston tragedy.
-- "Boston is not going to be intimidated by this," said Secretary of State John Kerry, a former U.S. senator from Massachusetts. "But we are going to find out who did this. The police work being done is extraordinary. The FBI is remarkable. There is a great deal of forensic evidence. We are hopeful we can bring people to justice."
"The Islamic Republic condemns killing of innocent people in U.S. Boston, Pakistan, Afghanistan, Iraq and Syria in equal measure," Iran's Ayatollah Ali Khamenei said Wednesday, according to a report on state-run Press TV.
-- Obama and first lady Michelle Obama will travel to Boston on Thursday for an 11 a.m. interfaith service dedicated to the victims. The president is expected to meet with the families of those wounded or killed.
--Obama declared an emergency in Massachusetts and ordered federal aid to supplement state and local response efforts.
CNN"s Steve Almasy and Faith Karimi contributed to this report from information provided by CNN staffers.