FBI and Homeland Security agents on Tuesday carried out searches at a high-rise apartment building in Boston seeking clues to unravel the plot behind the twin blasts, suspected to have been carried out by home-grown terrorists that killed three people and injured over 140 during the Boston Marathon.

Early on Tuesday, investigators descended on a high-rise apartment building in Revere and conducted a search, for nearly nine hours, related to the investigation into the first large-scale bombing in US since the 9/11 attacks. “FBI and Homeland Security agents were seen entering the Water’s Edge apartment complex at 364 Ocean Ave,” Boston Globe reported.

The Revere Fire department wrote on its Facebook page that this was a search for “a person of interest”.

CBS News reported that the apartment search was related to a man who is reportedly under guard at Brigham and Women’s Hospital. The man is a Saudi national who is in the US on a student visa. Several bags were removed from the scene around 2 a.m. on Tuesday, but authorities would not comment on the search.

President Barack Obama, who said any “responsible groups will feel the full weight of justice” did not call the explosions an act of terror but a little later a White House official said they were approaching the blasts as an act of terror.

Some terrorism experts warned that it will be “fool hardy” to speculate but other analysts believe that it was unlikely that an international terror network was behind the explosions.

Early reports suggest that the devices were crudely made - otherwise, they probably would have killed many more people - making it unlikely that they were the work of a foreign government or global terrorist group, such as alQaeda, the experts were quoted as saying by the Globe.

They said the Boston Marathon is an unusually public event, giving hundreds of thousands of people easy access to the 26-mile route, and the bombs could have been set off by radical Islamists from the US influenced by events or clerics in the Middle East, local extremists on either the left or right, or deranged killers with no ideological agenda.

Radical Muslims “will be everyone’s favourite suspect, but there are many other possibilities,” said Brian Michael Jenkins, a terrorism specialist at the Rand Corp.

The FBI issued a statement saying it “remains too early to establish the cause and motivation” of the attack, though the FBI agent overseeing the Boston office, Richard DesLauriers, called it a “potential terrorist investigation”.

The two blasts occurred within seconds on a sidewalk along the 42-kilometre route of the Boston Marathon, where thousands of people had lined up to cheer on the marathoners on Monday.

Hospitals reported at least 144 people are being treated, with at least 17 of them in critical condition and 25 in serious condition. At least eight of the patients are children.

Among the dead was an 8-year-old boy.

The authorities also found two devices at two other locations in the city, but they did not explode.

Several mega cities in the US, including New York, Los Angeles, Chicago and Washington, have been put on alert.

FBI special agent in charge Richard DesLauriers told reporters here that the investigation into the blasts is an “ongoing and criminal investigation that is a potential terrorist investigation.”

Media reports quoted a law enforcement advisory as saying that investigators have warned police to be on the lookout for a “darker-skinned or black male” with a possible foreign accent in connection with the attack.

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 near the finish line of the annual Boston marathon yesterday afternoon, CNN said.

President Obama was briefed on the explosions by Homeland Security adviser Lisa Monaco. Obama cautioned that “people should not jump to conclusions before we have all the facts.”

“But make no mistake; we will get to the bottom of this. And we will find out who did this and we’ll find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice,” Mr. Obama said.

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