Boston. The Federal Bureau of Investigation launched a manhunt on Monday for the perpetrators of a twin blast that killed three people and injured scores more near the finish line of the Boston Marathon.US officials said the criminal probe had the potential of becoming a terrorist one, drawing painful memories of the September 11, 2011 attacks that shocked the world."We still do not know who did this or why. And people shouldn’t jump to conclusions before we have all the facts. But make no mistake, we will get to the bottom of this,” President Barack Obama said in a briefing on Monday."We will find out who did this, we’ll find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice,” he added.Boston Police have so far confirmed that two explosive devices were detonated "simultaneously” in the heart of the city roughly four hours into the race, and near the finish line where runners, spectators and members of the media were crowded together.Bearings for maximum harmMore than 100 people were wounded by the devices, which were packed with gunpowder and shrapnel, according a senior law enforcement official briefed on the investigation who declined to be named, according to Reuters.Local police and doctors quoted by US media confirmed that ball bearings had ripped into those who were injured.At a press conference on Monday, the FBI declined to answer questions about the existence of other devices that had failed to detonate or had been defused by bomb squads.Some media reports have claimed that investigators found as many as five other bombs that did not explode.While some eyewitnesses claimed the blasts came from garbage cans along the marathon route, Boston Police Commissioner Edward Davis said investigators could not confirm if the bombs were hidden in mailboxes or trash cans.Searching for cluesDavis said authorities had received "no specific intelligence that anything was going to happen” at the race.As of yesterday morning, no arrests had been made and no individual or group had claimed responsibility for the attack.The New York Post reported late on Monday that a Saudi national was being questioned in handcuffs at a Boston hospital, but local police denied the story.Massachusetts Governor Deval Patrick asked Boston residents to give police any information that could advance the investigation.Reuters reported unnamed officials as saying investigators were considering at least two, but very contradictory, theories about who had organised the attack.One theory was that domestic extremists, such as anti-government anti-tax activists, were behind the attacks. This line of investigation was based on the timing of the explosions–Patriot’s Day, a public holiday in Massachussetts, which this year coincides with the deadline for US citizens to file their taxes nationwide.Several massacres involving home-grown terrorists, notably the 1993 Waco assault and the 1995 Oklahoma City bombing, occurred near or on Patriot’s Day.The second theory revolves around groups connected directly or indirectly to foreign-based Islamic militants. A senior law official who asked to remain anonymous, told Reuters that this theory was based on the form of the attack–two bombs in close proximity detonated in sequence at a high-profile event–a hallmark of certain jihadist groups.However, the Pakistani Taliban, who claimed the foiled Times Square car bomb in May 2010, yesterday denied any involvement in the deadly explosions."We are in favour of attacks against the United States and its allies, but we are not involved in the attack,” a Taliban spokesman told the AFP news agency.