Investigators warned last night that finding those responsible for the Boston marathon bombings would be a lengthy process likely to reach well beyond the city boundaries.
The FBI agent in charge said that while there were no early suspects, and Boston police confirmed that no-one was in custody, foreign terrorists could be behind Monday’s twin blasts that killed three and injured 176.
“This will be a worldwide investigation. We will go where the evidence and 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,” said special agent Rick DesLauriers.
“We continue to interview various witnesses and process the crime scene, which could take some time.”
Whether the attack originated overseas, or was the work of “domestic” terrorists will be the all-consuming question as detectives continue to pore over what Boston police commissioner Ed Davis said was “the most complex crime scene in the history of the city”.
Peter King, chairman of the Congressional committee on homeland security, said authorities were already considering possible suspects.
“If we take Islamic jihadists, a number of the most recent terrorists – the Times Square bomb, the attempted bombing of the New York subway – these were all domestic people with a foreign affiliation,” he said. “Al-Qaeda realises it’s difficult to get people into the country.
“Or it could be someone from a white supremacist group. All of this is being looked at.”
In a late-morning briefing, US president Barack Obama added: “What we don’t yet know is who carried out this attack or why, whether it was planned and executed by a terrorist organisation, foreign or domestic, or was the act of a malevolent individual. Clearly we’re at the beginning and it will take time to follow every lead and determine what happened.”
Publicly, the FBI is saying nothing. But agents and officers from the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) raided a flat belonging to a Saudi national in the Boston suburb of Revere and removed several large bags. The man, in the United States on a student visa, was one of those injured in the explosions.
In another development, authorities were still trying to locate the “dark-skinned” driver of a rental van denied access to a restricted area five minutes before the bombs went off.
Meanwhile, two anonymous senior officials at the Pentagon told CNN that investigators had not yet formed an opinion if domestic or international terrorists were responsible.
Experts say authorities are always guarded at the start of an inquiry, even if there are so-called “persons of interest”.
“It’s normal to see caution, you’re not going to make an arrest until you have much more evidence,” said Professor Rick Mathews, director of the National Centre for Security and Preparedness at New York’s University at Albany.
“But ‘person of interest’ says this is someone we are going to look at. Personally, I have a great deal of caution. It would surprise me if the person who did it would be that close to the device when it went off.”
Prof Mathews said fragments of the bombs that did go off would be invaluable in helping to identify the culprits, from providing fingerprints to confirming the “signature” of a known bomb-maker or group. “Just because the device wasn’t sophisticated doesn’t mean it wasn’t made by an outside terrorist organisation,” he said.