Nearly 20 hours after twin Boston Marathon bombings, law enforcement officials following up on “voluminous tips” described an active investigation Tuesday morning involving extensive photo and video evidence, but said no one is in custody.

During a press briefing at the Westin Copley, Boston Police Commissioner Ed Davis said the number of individuals injured in the Boylston Street bombings, which cut short the annual race and sent spectators fleeing the area while others raced to help, had risen to 176, with 17 people having critical injuries.

Three people, including eight-year-old Martin Richard of Dorchester, are reported dead so far. U.S. Sen. Elizabeth Warren said the other two fatalities were adults, and a spokeswoman for the Boston Police Department said she was waiting on the Medical Examiner’s Office to release the other identities.

Dr. Ron Walls, chairman of the Department of Emergency Medicine at Brigham and Women's Hospital, said at a separate briefing that objects resembling small ball bearings and nails and believed to be intentionally implanted projectiles were found in three patients treated while many more patients were injured by shrapnel caused by the bombings.

The bombings consumed the attention of elected officials Tuesday. Gov. Deval Patrick, Boston Mayor Thomas Menino, U.S. Sens. Warren and Mo Cowan, U.S. Rep. Stephen Lynch, U.S. Attorney Carmen Ortiz, Suffolk County District Attorney Dan Conley and officials from the F.B.I. and Bureau of Alcohol, Tobacco, Firearms and Explosives all attended the morning briefing at the Westin Copley.

The areas around the Copley crime scene perimeter were swarming with law enforcement presence, and SWAT teams were using parts of the Boston Common and Charles Street as a staging area Tuesday morning. An increased presence of Boston police officers were also noticeable throughout the Common.

In Washington, President Barack Obama pledged to “find whoever harmed our citizens and we will bring them to justice.”

“This was a heinous and cowardly act and given what we now know about what took place the FBI is investigating it as an act of terrorism,” Obama said. “Anytime bombs are used to target innocent civilians it is an act of terror. What we don’t yet know however is who carried out this attack or why, whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual.”

FBI Special Agent in Charge Richard DesLauriers said investigators are interviewing witnesses in various locations and their probe will have no geographic restrictions.

“This will be a worldwide investigation,” he said. “We will go where the evidence and the leads take us. We will go to the ends of the earth to identify the subject or subjects who are responsible for this despicable crime and we will do everything we can to bring them to justice.”

DesLauriers also addressed current safety considerations.

“There is no known imminent physical threat at any location where we might be conducting investigative activity right now,” DesLauriers said. “I want to put that out to the American public, to the citizens of the city of Boston and the Commonwealth of Massachusetts.”

DesLauriers said he was not aware of any threat information prior to the Boston Marathon.

U.S. Rep. Stephen Lynch, who suspended his U.S. Senate campaign after the marathon bombings, said he has known the family of the deceased 8-year-old boy Martin Richard for at least 25 years. The boy’s parents worked on his early campaigns, and Lynch said the mother worked with his wife at Marian Manor nursing home in South Boston.

Lynch, contrary to some published reports, said the father Bill Richard was not running in the marathon. The entire family went to Boylston Street to cheer on the runners and friends who were in the race, and were at the site of the second explosion.

“They were in the process of trying to actually get out into the street away from the buildings but that barrier that was to keep them out of the street was a problem and that’s when the second bomb went off,” Lynch said.

Lynch said he met with members of the family for hours Monday night, though some are in different hospitals. He described the youngest daughter as being in “grievous condition,” while the oldest son is dealing with the emotional trauma of watching his brother die and his family wounded.

“It shows how random this violence is that one son was taken and one was not and they were all there together watching this marathon,” Lynch said.

Lynch said Bill Richard still needed to be treated for shrapnel wounds to his legs, including what medical personnel confirmed to Lynch to be “ball bearings” included in the bomb at the site of the second explosion.

Bill Richard issued a statement on Tuesday through a spokesman: “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. 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.”

Rep. Kathi-Anne Reinstein (D-Revere) said she checked in with State and Revere police seeking information regarding the Ocean Avenue apartment in that city that law enforcement officials searched late Monday night. Reinstein told the News Service Tuesday that police were unable to give her any information because the investigation is being run by federal authorities.

Davis said special teams are reviewing a large quantity of photographic and video evidence of the area. “It’s our intention to go through every frame of every video that we have to determine exactly who was in the area,” he said. “This is probably one of the most well photographed areas in the country yesterday.”

The crime scene, described by Davis as the most complex in the history of the Boston Police Department, has been reduced from 15 to 12 city blocks. “We will continue to collapse that crime scene,” Davis said, though law enforcement officials said it could be several days before the Copley area is fully opened to the public.

While saying “we do have a threat,” Davis said there was no reason for people not to come to Boston. "We want people to come and go. We want you to life your life," he said. "We want you to be vigilant." Davis also said there was “no evidence” that the bombs were placed in trash can before they exploded.

DesLauriers, in response to a question about potential suspects, said, “I’m not going to say who might or might not be in custody right now.” Davis later said, “No one is in custody.”

ATF Acting Special Agent in Charge Gene Marquez said during the press briefing that rumors of seven explosive devices in the city on Monday were "not true" and that authorities are only aware of the two devices that exploded almost simultaneously at 2:50 p.m.

Gov. Patrick said officials were planning an interfaith prayer service for Wednesday.

“The world will never be quite the same,” Sen. Warren told reporters after the morning press briefing. “We have to find ways to fight back, and part of the way we fight back is by moving forward.”

“We cheer each other on. We carry each other across the finish line. And now we’re working together to survive a tragedy and I think that’s the best thing I can say about Boston. We’re all pulling together,” Warren continued.

Asked about comparisons to Sept. 11, 2001 and whether the bombings should be considered terrorism, Warren said, “We don’t know for certain was has happened. But this is clearly an act of terror and at this point we’re pursuing every possible lead both nationally and internationally. We will find out who did this and we will bring them to justice.”

At the quiet State House Tuesday morning, the House and Senate observed moments of silence in memory of the victims of Monday’s bombings. The House adjourned without naming negotiators to work with the Senate on a compromise $500 million tax bill approved by the Senate early Saturday night.

U.S. Sen. Mo Cowan was in Washington at the time of the blasts meeting with constituents. He returned to Boston Monday evening after receiving an “outpouring of support” from his Senate colleagues, who observed a moment of silence on the Senate floor Monday.

“It was a powerful moment,” Cowan said. Asked about the coordination between federal, state and local investigators, Cowan said, “There seems to be smooth cooperation and that’s encouraging and I think that means we’ll get to answers as quickly as we can. Everyone’s rowing in the same direction.”

The five candidates in the April 30 U.S. Senate primary campaigns released statements Monday suspending their campaigns. On Tuesday, Republican candidate Gabriel Gomez said he finished the race minutes before the blast and expressed gratitude that his family and staff were unharmed.

“Even in the face of unspeakable cruelty the people of Massachusetts immediately came together to help in any way they could,” Gomez said in a statement. “From the first responders I saw run towards the blast, to the neighbors who opened up their homes to frightened runners, the worst evil of the world has brought out the best in Boston.

“Today is not a day for politics, but I’d like to thank Mike Sullivan and Dan Winslow for calling immediately after the attack to make sure that we were clear of the danger. To think about the kind of person that would plant bombs at the feet of children to maim and kill, my emotions have boiled over from sorrow, to grief, to anger. As a military veteran I’ve seen awful things, but the scenes from the Back Bay yesterday were almost too much to bear. We will find who is responsible for these deadly acts and make sure justice is swift and clear.”

Rep. Sarah Peake, a Provincetown Democrat, told the News Service Tuesday she attended Monday’s Red Sox game, and was heading toward the marathon finish line when she heard the bomb blasts. Peake and her wife, Lynn Mogell, stopped near Massachusetts Avenue to cheer on the runners, when they heard two loud booms, and “could feel the reverberations in your chest,” she said.

“We just looked at each other, and didn’t say a word,” she said.

In the seconds following, Peake said, she watched a State Police trooper standing across the street to see if he reacted. A moment later, she saw a “sea” of police cars racing toward the finish line, and police officers on foot running alongside marathon runners, racing past them toward Copley Square.

“It was just a massive public safety rush up toward Copley,” she said.

Peake said she had friends watching the marathon near where the bombs went off, who left only minutes before the blast. After leaving the House chamber Tuesday, Peake said she felt “emptiness” similar to the way people felt after the 9/11 attacks.

Rep. Donald Wong, a Saugus Republican, was a block away from the blasts watching the marathon with his wife and family, including his 20-month-old granddaughter Ava. Wong said people started running.

“I think a lot of people just didn’t know what was happening, and those who did were trying to get away,” he said. Wong said he was concerned for his family’s safety, and kept his granddaughter close to him because of all the people running by. “This is something that is unnatural to us here,” Wong said.

On Tuesday, Wong was headed to visit a Wakefield mother, Liz Norden, whose two sons who each lost a leg during the explosions. He said he wanted to offer his support, and see if he could “expedite” any help they might need.

During Tuesday’s session, Senate President Therese Murray noted in particular the death of Martin Richard, the 8-year-old boy from Dorchester, and injuries sustained by his family.

“Our community is devastated and our hearts are broken,” said Murray, who said her office has received condolences from Ireland, Russia and Australia.

A Utah woman still in her marathon outfit sat in the gallery, and later said she had been unable to finish the race because of the bombing and was in the midst of a tour of the Freedom Trail on Tuesday.

[Colleen Quinn and Andy Metzger contributed reporting]