(CNN) - Boston area hospitals received 183 patients who sustained injuries in Monday's attack, according to the latest CNN tally. By Tuesday afternoon, at least 89 of those people had been released.
A senior U.S. official told CNN, "There is no reporting indicating a foreign connection, or any reaction from al Qaeda." The official said that is based on information circulated through senior levels of the administration earlier Tuesday, and it could change as the investigation progresses.
An injured Saudi student who was questioned after the bombing was a spectator at the race and was running away from the scene because of his injury, another U.S. official told CNN. The student was interviewed by authorities "but it came up empty. He was just in the wrong place at the wrong time."
Three people died and scores were hurt in two bomb blasts, 12 seconds apart, that erupted near the finish line of Monday's Boston Marathon, police said.
At least nine of the wounded are children.
The Department of Homeland Security issued an advisory about pressure-cooker bombs in 2004. Bombs are made by placing TNT or other explosives in a pressure cooker and attaching a blasting cap at the top of the pressure cooker, the advisory said. "The size of the blast depends on the size of the pressure cooker and the amount of explosive placed inside. Pressure cooker bombs are made with readily available materials and can be as simple or as complex as the builder decides. These types of devices can be initiated using simple electronic components including, but not limited to, digital watches, garage door openers, cell phones or pagers."
President Barack Obama said officials do not know whether the bombing was the work of a terrorist group or "a malevolent individual," nor do authorities have a sense of what may have been the motive.
"We will find whoever harmed our citizens and we will bring them to justice," he said.
No one is in custody, Boston Police Commissioner Ed Davis told reporters Tuesday.
Authorities are processing "the most complex crime scene that we have dealt with in the history of our department," Davis said.
Two explosive ordnance disposal sweeps were carried out Monday, the first early in the morning and the second an hour before the first runners crossed the finish line, Davis said. "They did not turn up any evidence," he said.
"Make no mistake: An act of cowardice and of this severity cannot be justified or explained," Suffolk County District Attorney Dan Conley said. "It can only be answered."
Thirty forensic specialists and a number of dogs trained to detect explosive devices and their residue are at the scene of the blasts, according to Gene Marquez, special agent in charge for the Bureau of Alcohol, Tobacco, Firearms and Explosives.
No unexploded devices were found, Marquez said.
Authorities are asking those who may have video or pictures from the scene around the time of the blasts to call city or FBI hot lines.
The blast site will take several days to process, Marquez said.
FBI Agent Rick DesLauriers said law enforcement has received a "voluminous" number of tips.
A law enforcement official said there was no specific suspect in the bombings and no leading theory on a motive.
The official said investigators have found no surveillance video showing the bombs being put in place but were combing through video from nearby businesses and other places.
The intelligence community is poring over all threat reporting to see if there is anything that could be connected to the explosions in Boston, U.S. counterterrorism officials said.
The federal Emergency Response Team is beginning to inventory the evidence, a federal law enforcement source said.
The device may have been placed in a trash can, from which shrapnel would have been created when it detonated.
Authorities searched an apartment late Monday in the town of Revere, northeast of Boston, and removed items, but would not say how the search might be linked to the investigation.
The official said the Revere search is connected to a young Saudi man on a student visa who has been questioned at a hospital.
The search found nothing related to the bombing, the official said. It took place with consent, so no search warrant was needed, a federal law enforcement official told CNN.
U.S. and Saudi officials said more than one Saudi has been interviewed. CNN knows of a male, whose apartment was searched, and a female. All those interviewed are cooperating, and none has been labeled a suspect, Saudi officials, said.
At least two Saudi nationals were wounded in the Boston Marathon bombings, a Saudi Embassy official told CNN. Neither is a suspect in the attack, the Saudi official said, citing U.S. officials.
One is a student, 20, who suffered shrapnel wounds and was questioned as a possible witness, the source said. A U.S. official told CNN the student "was just in the wrong place at the wrong time."
The other is a medical student who was at the marathon with her husband and child, the source said.
Investigators have urged police to be on the lookout for a "darker-skinned or black male" with a possible foreign accent in connection with Monday's bombings, according to a law enforcement advisory obtained by CNN. The man, seen with a black backpack and sweatshirt, was trying to enter a restricted area about five minutes before the first explosion, the notice says.
The FBI has taken the investigation's lead role, said DesLauriers, the special agent in charge of the bureau's Boston office.
"The situation remains fluid, and it remains too early to establish the cause and motivation," the FBI's Boston Division said in a statement asking people to call in with any information, images or details related to the explosions.
The Pakistani Taliban was not involved in the attack, spokesman Ihsanullah Ihsan said.
Federal law enforcement has been placed on "Level 1 mobilization," U.S. government sources said. "That's equivalent to all hands on deck," one official said. A senior federal official told CNN that teams were on standby to search flights leaving the United States; no team had been activated.
A knowledgeable source called the bombs "crude."
The bombs appear to have been inside metal pressure cookers that were in backpacks, a federal law enforcement official with knowledge of the investigation told CNN.
A law enforcement official said it was "likely, but not certain" that a timing device was used and that at least one of the bombs was not activated by a cell phone.
In all, 183 people were treated at area hospitals after the blasts, according to a CNN Tally. At least 23 were hospitalized with critical injuries, at least 40 received serious injuries.
Of them, 44 patients were treated at Brigham and Women's Hospital and Brigham and Women's Faulkner Hospital, said Dr. Ron Walls, chairman of the Department of Emergency Medicine.
Three patients had been struck with metal beads slightly larger than BBs, he said. Doctors removed more than a dozen small nails from one patient, he said.
Virtually the entire trauma surgical unit had been on duty at the time of the blast, he said.
Martin Richard, 8, was killed. His mother, Denise, underwent surgery for a brain injury, and his 6-year-old sister lost her leg, WHDH reported.
"I ask that you continue to pray for my family as we remember Martin," the boy's father, Bill Richard, said in a statement. "We also ask for your patience and for privacy as we work to simultaneously grieve and recover."
Also killed was Krystle Campbell, a 2001 graduate of Medford High School, Medford Mayor Michael McGlynn said.
Eight of the 31 patients at Massachusetts General Hospital were in critical condition late Monday, trauma surgeon Peter Fagenholz said. The most serious wounds "have been combined, complex lower injuries that involve blood vessels, bone and tissue," and several underwent amputations, he said.
The bombings resulted in at least 10 amputations and left doctors picking ball bearings out of victims in the emergency room, a terrorism expert briefed on the investigation said.
In addition to the patients treated at Massachusetts General, 33 patients were treated at Beth Israel Deaconess Medical Center; 23 at Boston Medical Center; 18 at Tufts Medical Center; 31 at Brigham and Women's Hospital and 13 at Brigham and Women's Faulkner Hospital;10 at Boston Children's Hospital; five at Carney Hospital; 18 at St. Elizabeth's Medical Center; and one at Newton-Wellesley Hospital.
Officials in other cities, including London, Washington, New York, Miami, Chicago and Los Angeles, said they were monitoring events and had stepped up security.
London Met Police Chief Superintendent Julia Pendry said: "A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon." That marathon is scheduled for Sunday.
"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 Tuesday at the White House after meeting with key advisers about the Boston Marathon bombing.
The flags at the White House, like those on Capitol Hill on Monday, were lowered Tuesday to half-staff.
In a telegram to Cardinal Sean O'Malley, archbishop of Boston, a spokesman for Pope Francis said, "At this time of mourning, the Holy Father prays that all Bostonians will be united in a resolve not to be overcome by evil, but to combat evil with good (cf. Rom 12:21), working together to build an ever more just, free and secure society for generations yet to come."
A number of professional sports teams held moments of silence Monday night.
The Virgin London Marathon will mark 30 seconds of silence before this Sunday's race and is urging runners to wear a black ribbon to mark the Boston events.
Saudi Ambassador to the United States Adel bin Ahmed Al-Jubeir condemned the bombings.
"What occurred ... in Boston is a heinous crime which contradicts the values of humanity," he said.