Lone terrorists hard to spot: US experts The Boston bombings have highlighted the almost impossible task US law enforcement face in preventing terror attacks say security experts. This is especially so when individuals act in isolation, they say. Despite evidence that one of the suspects, Tamerlan Tsarnaev, had come to the attention of US authorities prior to last week's bomb blasts, analysts warn attempting to thwart similar attacks in the future may prove futile. Tsarnaev, 26, is suspected of carrying out the April 15 attacks with his brother Dzhokhar, 19. The bombings killed three people and injured 264. Tamerlan was killed during a bloody gun battle with police on Thursday as the net closed in on the two siblings of Chechen descent. Dzhokhar, who remains under guard in a Boston hospital, has reportedly told investigators his older brother was the driving force behind the attacks and "wanted to defend Islam from attack." The teenager, who faces life imprisonment or the death penalty after being charged for his role in the attack, has also told investigators the brothers received no financial support or assistance from any foreign group. The 19-year-old also said he and his brother killed an MIT campus police officer on Thursday night, according to one source. He made his statements at Beth Israel Deaconess Medical Center. Dzhokhar'S health is apparently improving - the hospital listed his condition as fair on Tuesday. He was formally charged with using weapons of mass destruction against people in detonating two bombs on April 15 near the finish line of the Boston Marathon. Dzhokhar said he and his brother were driven to the attack by jihadist radicalism sparked by the US wars in Iraq and Afghanistan in which thousands of Muslims have died, a federal law enforcement official said. According to counter-terrorism experts, the mounting probability that the Tsarnaevs may have been radicalised via militant Islamic sources on the internet is more alarming than the possibility they were part of a wider plot orchestrated by a group such as al-Qaeda. Tamerlan Tsarnaev had showed increased signs of radical behaviour at the mosque he attended in Cambridge, Massachusetts, and he had been quizzed by the FBI at the request of Russian authorities in 2011. He later travelled to Dagestan for five months, and US Homeland Security Secretary Janet Napolitano acknowledged to lawmakers on Tuesday that security monitors "pinged" when he left. But Republican Senator Lindsey Graham had said earlier that an FBI official told him Tsarnaev's trip was not flagged to the agency because his name was misspelled, possibly on a plane ticket. Even still, little could have been done because at that point he had not committed any crime, counter-terrorism expert Philip Mudd, a former deputy director of national security at the FBI said. "We've got a Constitution that says you can be a radical, speak however you want to speak," Mudd said. Even if we wanted to, we could not investigate every radical in this country. Tracking every individual with radical views in the United States is impossible, the Brookings Institution expert added. Mudd's view is shared by Joseph Young, an assistant professor at American University's justice, law and society department. "The really unsettling part is that it's really difficult to predict who's likely to do these kinds of things," Young said. The brothers' parents are being questioned in Dagestan by US embassy staff and the FBI.