2013 Boston Marathon explosion

President Barack Obama declared Monday's Boston Marathon bombings an "act of terror," while authorities said that no suspect is in custody for the attack that killed at least three people and injured 176, with 17 still in critical condition.

"We will find whoever harmed our citizens and we will bring them to justice," Obama said in a press conference Tuesday morning. Obama spoke shortly after ordering flags at the White House and other government buildings to be flown at half-mast in "a mark of respect for the victims of the senseless acts of violence perpetrated on April 15, 2013."

"Any time bombs are used to target civilians, it is an act of terrorism," Obama said, adding that it remained unclear who was behind the attack and why.

In an earlier FBI press conference on Tuesday morning, Boston Police Commissioner Ed Davis said that no one is in custody and that law enforcement officials are following up on tips. He urged the public to send in any photos or videos taken near the scene to look for clues. FBI Special Agent in Charge Rick Deslauriers added that "this will be a worldwide investigation" and that there are "no known additional threats." Authorities did not speculate on a motive.

Additionally, Massachusetts Gov. Deval Patrick said that the other suspicious packages found Monday were not explosives, refuting reports that devices were found and deactivated.

Police identify two bombing victims

As of now, the crime scene has been reduced from a 15-block radius to 12 blocks as they slowly work through "the most complex crime scene we have dealt with in the history of the department," Davis said. Processing the scene will take several more days.

"This is a bad day for Boston, but I think if we pull together we'll get through it," Mayor Tom Menino said, noting the resilience of all those involved. "Boston will overcome."

In a separate press conference Tuesday morning, Dr. George Velmahos, the chief of trauma surgery at Massachusetts General Hospital, said that eight patients, all of whom he believes were spectators, remain in critical condition at the hospital and range in age from 28 to 71. Four patients have already undergone amputations with two more with limbs "at risk." Velmahos said that most of the patients are in a stable condition, though some remain in a medical coma. He added that he hopes the first patients will be ready to be released in a few days, but it's too early to know for sure.

Velmahos said that he suspects the bomb contained shrapnel. Some patients suffered 20 to 40 shrapnel wounds, with most of the fragments "nail-like" or "pellets."

Law enforcement officials previously confirmed three fatalities on Monday, including an 8-year-old boy. The Boston Globe has identified the child as Martin Richard, who was cheering on his father, Bill, in the marathon with his family. His mother, Denise, is recovering from surgery for a brain injury, and his younger sister lost a leg, according to local affiliate WHDH. His older brother was reportedly uninjured.

The explosions happened at about 2:45 p.m. ET Monday near the finish line, about an hour after the first runners completed the race. The 26th mile of the race was dedicated to the victims of the shooting at Sandy Hook Elementary School last year.

According to CNN, a Saudi national with a leg wound was under guard at a Boston hospital in connection with the bombings as of late Monday evening, but investigators have not commented on specifics of the investigation, including suspects and possible leads. Earlier in the evening, investigators also reportedly sent a law enforcement advisory warning authorities to be on the lookout for a "darker-skinned or black male" with a possible foreign accent in connection with the bombings. The man was seen with a black backpack and sweatshirt and had been trying to get into a restricted area about five minutes before the first explosion.