(CNN) -- Authorities including bomb experts searched an apartment in Revere, Massachusetts, and removed items, after two deadly bombs struck the Boston Marathon. But officials cautioned that there are currently no clear suspects -- and the motive remains unknown.
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. But investigators announced Tuesday that there were only two bombs -- the two that exploded at the marathon, killing three people and wounding 176.
Doctor believes bombs contained sharp objects
The bombs that turned a celebration into a nightmare 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.
A doctor at Massachusetts General Hospital, where some of the injured are being treated, told reporters Tuesday that patients had nails -- or sharp objects that were like nails -- and "pellets' inside their bodies. The bombs probably contained metallic fragments, Dr. George Velmahos, head of trauma care, said at a news conference.
Asked whether what was found in the patients' bodies could have come from nearby objects that exploded in the blast, Velmahos said he believes the materials were likely part of the explosive devices.
Authorities have not said what the bombs may have been made of composed of.
Tiny clues at the crime scene may provide those answers. Investigators are carrying out the painstaking process of analyzing fragments for anything that could indicate the bombs' "signature," said the federal law enforcement official who works in the intelligence community.
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.
Boston's 'most complex crime scene' ever
The crime scene has been reduced from 15 blocks to 12 and will be narrowed as the investigation proceeds, Davis said Tuesday.
He described it as "the most complex crime scene that we've dealt with in the history of our department."
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.
The large number of photos and videos from the marathon will keep numerous investigators busy.
Davis vowed authorities will sift "through every frame of every video."
Authorities have asked anyone with images from any part of the marathon to share them with police.
"People don't know that they were witnesses -- that they might actually have evidence in their phones or in their cameras," Juliette Kayyem, President Obama's former assistant secretary for homeland security, said on CNN's "Starting Point."
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.
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 Kayyem, who also served as homeland security adviser to Gov. Patrick.
The move minimized chaos and "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.
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.
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.
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.
CNN's John King, Matt Smith, Steve Almasy, and Monte Plott contributed to this report.