“We have nothing to hide,” said Mr. Khadzhimuratov, who has not hired a lawyer. “But they began very nice, saying they needed an expert on the North Caucasus. Now they treat me like a criminal. They push, push, push. They say, ‘Where do you think he made the bomb? It took 12 seconds to go off — how do you think they set off the bomb?’ ”

The F.B.I. declined to comment. But aggressive and accusatory questioning may simply reflect its determination to be absolutely certain that the bombing conspiracy did not extend beyond the Tsarnaev brothers. Investigators have found no link to foreign militant groups, despite Tamerlan Tsarnaev’s travel to Russia’s Caucasus region last year.

The homegrown theory received additional support on Thursday when officials confirmed a report by CBS News that Dzhokhar Tsarnaev had scribbled an explanation of the bombing before his arrest on April 19.

After the death of Tamerlan in a police shootout, Dzhokhar hid inside a neighbor’s boat and wrote with a pen on the inside of the hull that the attack was retribution for the wars the United States waged in Iraq and Afghanistan, according to two law enforcement officials. The note stated generally that an attack on one Muslim is an attack on all Muslims, one official said.

In Senate testimony on Thursday, the F.B.I. director, Robert S. Mueller III, suggested that the evidence pointed to “homegrown violent extremists,” but he said investigators “continue in our ongoing efforts to identify any others who may be responsible.”

The note written on the boat, which is likely to become evidence at any trial for Dzhokhar Tsarnaev, fits with his statements to investigators that the brothers were inspired by calls for jihad against America, not the anti-Russian insurgency in the Caucasus.

But the F.B.I. has worked closely with Russian security officials to understand what Tamerlan did between January and July 2012 in Dagestan in southern Russia, where he reportedly sought to meet with militants.