Dozens of federal agents and local and state police officers are tracing the steps of the Tsarnaev brothers in the weeks and months before the Boston Marathon bombing, but they have not been able to connect them to a foreign terrorist organization, according to law enforcement and intelligence officials.

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 with militants 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.

Tracing the 9mm Ruger handgun has been difficult because the serial number was erased. But agents were able to partly raise the number and are working on a handful of possible leads, law enforcement officials said.

FBI agents, working out of Boston’s Joint Terrorism Task Force and traveling to other U.S. cities and abroad, are scouring computer, financial, phone and travel records to learn all they can about the activities of Tsarnaev and his brother, Dzhokhar, before they allegedly detonated two pressure cookers filled with explosives at the finish line of the Boston Marathon on April 15. The agents are also conducting interviews with anyone who may have come into contact with the brothers in the United States or abroad.

“We are trying to determine the full story of this crime,” FBI spokesman Paul Bresson said. “Anything and everything we can find out about it. There are still many, many questions.”

Dzhokhar Tsarnaev, who suffered gunshot wounds and was captured, told FBI investigators before he was charged that he and his brother made the bombs in the apartment in Cambridge, Mass., that his brother shared with his wife, Katherine Russell, and their daughter. A law enforcement official said Wednesday that Dzhokhar told investigators that his sister-in-law was not involved in the plot.

Russell has not been charged, and her lawyer has said she was shocked by the bombing. An FBI spokesman said the bureau is still investigating whether she was involved.

Even as that criminal probe proceeds, there is a parallel effort to produce a multi-agency assessment of the “radicalization” of the Tsarnaevs, officials said.

The 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.

Officials seeking to reconstruct the plot said the Tsarnaevs may have left fewer clues because they appear not to have communicated extensively with each other about the alleged plan, or with other individuals.

“The problem here is you’ve got two brothers,” the official said. In other cases, including the shooting at Fort Hood, Tex., in 2009, the suspect was accused of communicating with al-Qaeda operatives overseas or seeking other direction and help in the attack.