House committee hearing on Boston bombings Thursday, as investigators continue to trace activities of Tsarnaev brothers

The House Committee on Homeland Security will hold a hearing Thursday on the deadly bombings, which killed three and injured more than 200. Rep. Michael McCaul (R-Tex.), the committee’s chairman, called for the hearing to investigate and review what U.S. agencies knew about the alleged bombers before the attacks.

Some reports have suggested that one of the brothers, Tamerlan Tsarnaev, met withmilitants in the strife-torn region of Dagestan last year during his six months in Russia. But one U.S. official, speaking on the condition of anonymity, said that issue was “still in the category of question marks.”

At the same time, agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives are trying to trace the gun that Tsarnaev allegedly used in a gunfight with police before he was killed April 19. They are hoping that identifying the first purchaser of the gun could shed light on where Tsarnaev obtained the firearm.

A U.S. official said that the FBI, the National Counterterrorism Center and the Department of Homeland Security are developing a formal intelligence assessment on the factors that moved the Tsarnaevs toward hard-line Islamist views, and whether there was a single development or tipping point in their alleged turn to violence.

“We need to understand it to counter it,” the official said. “From that we look at how do you put a brake in the radicalization process, and can you put something in that path to detect it.”

The official said the research, which involves experts on radicalization at NCTC and other agencies, is expected to take several months, culminating in a formal intelligence assessment that could be distributed across the executive branch.