The FBI says they have no suspects nor anyone in custody over the deadly Boston blasts. Police are calling on witnesses to assist the investigation which has been called “the most complex crime scene” in the history of the Boston PD.

While the investigation has yielded no results, with no one claiming responsibility, more details have emerged on the nature of the explosives.



According to a person close to the Boston Marathon investigation, the explosives were in 6-liter pressure cookers and placed in black duffel bags. The person, who spoke on the condition of anonymity, said investigators have some of the bomb components but did not yet know what was used to set off the explosives.



The statement echoes the Massachusetts General surgeon’s words that given the injuries they treated, "most of [the metallic fragments in the patients] were in the bomb."



The General surgeon addressed the media Tuesday. He said that the shrapnel was consistent in size, and included nail-like fragments.



"There were people who had 10, 20, 30 of them in their body," he said.



Earlier, FBI and Boston city authorities declined to comment on the nature of the explosives that were triggered 12 seconds apart in the late hours of the race, when most of the top runners had finished and ordinary participants were completing the event.



Massachusetts Gov. Deval Patrick stressed that “only two explosive devices were found yesterday”, rejecting reports that earlier suggested five explosives were found.



Addressing the public the morning after the tragedy, FBI, Boston city authorities and police said that they are still seeking public assistance, calling for witnesses to come forward.



“We are processing the most complex crime scene in the history of our department,” Police Commissioner Edward Davis said.



People who captured amateur photos and videos are being asked to submit their footage as it might help with the investigation into the double blasts which have killed three and left more than 170 people injured.



"We have received voluminous tips over the last 18 hours since the incident," FBI special agent in charge, Rick DesLauriers said.



The police gave no details on the going investigation.

Police called on citizens to be patient as they will see a "continued, heightened police presence".

Police are searching the 26.2 mile route of the race. Currently the crime scene has been scaled down to12 blocks. FBI special agent in charge, Rick DesLauriers, said that there is currently “no known physical threat."

US Homeland Security Secretary Janet Napolitano said Tuesday there was no indication that the bomb blasts in Boston were part of a broader plot.

"While there is no current indication to suggest that the events in Boston are indicative of a broader plot," she said in a written statement. "Out of an abundance of caution, DHS continues to keep in place enhanced security measures at transportation hubs, utilizing measures both seen and unseen."



Investigators have searched an apartment in the Boston suburb of Revere, which police confirmed to be connected with the blast probe. But early reports of a detained suspect were later said to be false.



Authorities gave no details on the search. However, investigators were seen leaving a building there early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag, AP reported.



Officials declined to comment on the suspects. When asked if there was anybody in custody, Police commissioner Ed Davis said "No-one's in custody."



The FBI special agent DesLauriers also declined to comment saying he is “not going to say who might or might not be in custody right now."



Earlier on Tuesday reports said police are seeking a suspect in the deadly Boston blasts described as a “darker-skinned or black male” with a backpack, media reports. They are also looking for a yellow van.



The suspect, who is said to possibly have a foreign accent, was seen trying to enter a restricted area five minutes before the first explosion, CNN reported.



As he was not allowed to do so, the man reportedly walked away and pulled his sweatshirt hood over his head.



When asked about a bomb placed in a trash can, Police commissioner Ed Davis said there was no evidence of that.

So far no organization or individual has claimed responsibility for the two explosions that went off at the finish line of the signature sporting event.



Investigators have not voiced any official theories as to who could be behind the attack, only saying that they're treating it as a potential act of terror.

Addressing the nation for the second time in 24 hours following the tragedy, President Barack Obama said the deadly Boston Marathon bombings “were an act of terrorism”. He said investigators do not know if they were carried out by an international or domestic organization, or perhaps by a

"malevolent individual." Unofficially, two possibilities have been suggested. One points the finger at domestic right-wing radicals, who may have timed the attack with Patriots' Day in the US. Monday was also the day when tax returns are due in America, which may also see a link to anti-government extremists.

Another version put the blame on Islamist militants. Planting multiple explosive devices at a large public event or a public venue like a marketplace is a common tactic for terrorist attacks in Afghanistan, Pakistan, Iraq and Yemen. The Pakistani Taliban has denied any role in the bombings.

There are usually some 27,000 runners at the annual event and about 500,000 people come to watch it. Officials expect to analyse vast amounts of visual evidence of what happened before and after the explosions.