Officials also announced a  twist in the probe: Suspicious packages that were detonated out of  precaution were not explosive devices after all.

After the blasts Monday,  some officials reported that explosive devices that failed to go off  were found. U.S. Rep. Bill Keating of Massachusetts, who serves on the  Homeland Security committee, said there were two.

But Massachusetts Gov.  Deval Patrick announced Tuesday that there were only two bombs -- the  two that exploded at the marathon, killing three people and wounding  154.

Saudis interviewed

The search on the home in  Revere, north of Boston, was connected to a young Saudi citizen who is  visiting on a student visa and has been questioned, a law enforcement  official said. So far, the official told CNN, he has not heard of  anything being found connecting the person to the bombings.

The Revere Fire  Department said on its Facebook page that the FBI; the Federal Bureau of  Alcohol, Tobacco, Firearms, and Explosives; immigration officials,  state and local police, detectives and bomb techs all took part in the  search at the apartment.

The search took place with consent, so no warrant was needed, a federal law enforcement official said.

Investigators told police  Monday to be on the lookout for a "darker-skinned or black male" with a  possible foreign accent in connection with the marathon bombs,  according to a law enforcement advisory obtained by CNN. The man was  seen with a black backpack and sweatshirt and was trying to get into a  restricted area about five minutes before the first explosion, the  lookout notice states.

A Saudi citizen with a  leg wound was under guard at a Boston hospital in connection with the  bombings, but investigators cannot say whether he is involved at this  time, and he is not in custody, a law enforcement source said.

A Saudi woman, a doctor, has also been interviewed, according to a law enforcement source.

Boston Police Commissioner Ed Davis said many people were being questioned.

Authorities also plan to  search through videos from surveillance cameras near the attack in  Boston's Copley Square. So far, no footage has been spotted showing  someone placing the bombs, a law enforcement source said.

Nothing ruled out

The intelligence community is pouring through all threat reporting for any clues, U.S. counterterrorism officials told CNN.

That includes any claims made on jihadist websites.

Nothing is being dismissed this early on, the officials said.

It isn't clear Monday whether the origin of the bombings was domestic or foreign.

Small fragments could yield big clues

Tiny clues may help lead  to who was behind the terrorist attack that killed three people and  wounded 152. Investigators are beginning the painstaking process of  piecing through fragments for anything that could indicate the  "signature," said a federal law enforcement official who works in the  intelligence community.

The bombs were small,  and initial tests showed no C-4 or other high-grade explosive material,  suggesting the packages used in the attack were crude devices, a federal  law enforcement official in the intelligence community said.

Keating called the bombings a "sophisticated, coordinated, planned attack."

A law enforcement  official in Boston said investigators "have a number of active leads and  some good early progress in the forensics analysis."

There were no credible threats ahead of the race, a state governmento fficial said.

The FBI is taking the lead in investigating the attack near the marathon's finish line.

"This will be a combined  federal, state and local effort," Richard DesLauriers, the special  agent in charge of the FBI's Boston Division, said at a news briefing by  law enforcement officials on Monday evening.

Describing it a  "criminal investigation" that is also "a potential terrorist  investigation," DesLauriers said the FBI was declaring federal  jurisdiction over the matter through the Boston Joint Terrorism Task  Force.

Quick action helped preserve crime scene

Boston officials who  worked quickly Monday to clear the crime scene and divert thousands of  runners half a mile away should get an award, said President Obama's  former assistant secretary for homeland security.

The move minimized  chaos, said Juliette Kayyem, who also served as homeland security  adviser to Gov. Patrick. She told CNN's "Starting Point" on Tuesday that  it also "preserved the crime scene, which is going to be key for the  FBI investigation. Those are lessons learned out of 9/11."

Open events are hard to  secure, Kayyem said. "People say, 'Oh, how could this happen again?...'  The better way to look at it, I think, is: Did we respond better? I  think the answer is yes."

Police are also taking  other steps -- including asking people at Boston's airports to check  their phones. "People don't know that they were witnesses -- that they  might actually have evidence in their phones or in their cameras,"  Kayyem said. Anyone among the thousands of people who traveled to Boston  and were anywhere near the marathon may have a photo or video of a  perpetrator and not realize it.

"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.

"No piece of information or detail is too small," it said.

A celebration turns into a bloody scene of destruction

Doctors remove ball bearings from victims

Despite the lack of answers at this point, officials vow to find whoever was behind the attacks.

"Make no mistake, we will get to the bottom of this, and we will find out who did this," President Barack Obama said Monday.

"Any responsible individuals, any responsible groups, will feel the full weight of justice," he added.

Witness: 'I saw blood everywhere'

Cell phone towers could offer clues

In addition to  scrutinizing images of surveillance cameras in the area, the FBI was  most likely issuing subpoenas for records from cell towers in the area  to isolate and trace calls from around Copley Square at the time of the  blasts, according to a federal law enforcement official.

Bombing is the latest Patriots Day tragedy

As authorities searched  the scene, numerous suspicious packages were found, possibly because  people fled the area, leaving items behind. Investigators were checking  those objects.

Bomb-sniffing dogs were  working the area of the bombings and nearby streets, checking every item  on curbs or near the street -- including "every construction cone,  every Port-a-Jon" -- to make sure there were no explosive devices left, CNN affiliate WHDH in Boston reported.

After initial  suggestions that a third blast Monday, which took place at the John F.  Kennedy Presidential Library, could be related to the marathon bombings,  police said that that incident was connected to a mechanical problem.

The library said all staff and visitors were safe.