BOSTON — A congressional committee’s investigation into the Boston Marathon bombings has reinforced that the biggest threat to the U.S. and other countries is the radicalization of homegrown terrorists, U.S. Rep. William Keating said Monday.

He said the Homeland Security Committee is close to wrapping up its report on the April 15 bombings, which killed three people and injured more than 260 others near the race’s finish line, and it should be released in early February.

The Massachusetts Democrat spoke by phone Monday from Moscow, where he and House Homeland Security Committee Chairman Michael McCaul, R-Texas, were talking with government officials and others about the bombings. They planned to head to Sochi on Tuesday to review security procedures and threats ahead of the Winter Olympics.

Keating said it appears bombing suspect Tamerlan Tsarnaev met with insurgents in Russia but didn’t pass their vetting process and instead returned to the U.S. Tsarnaev was killed in a shootout with police a few days after the bombings. His brother, Dzhokhar Tsarnaev, has pleaded not guilty to a federal indictment charging him with using a weapon of mass destruction. Aside from three friends of Dzhokhar Tsarnaev’s accused of hindering the investigation, no one else has been charged in connection with the attacks.

Russian officials sent a letter to the FBI in March 2011, warning that Tamerlan Tsarnaev, an ethnic Chechen, had plans to join insurgents in Chechnya. The FBI opened an investigation, but the case was closed when no evidence of terrorism was found.

In January 2012, Tsarnaev spent six months in Dagestan, a southern Russian province that has become the center of an Islamic insurgency and where his family lived for a time before moving to the U.S. One question in the bombing investigation has been whether he was radicalized there.

“As much as he did come back to Russia, as much as he did meet, at least I believe, with insurgents, the vetting process wasn’t successful for him, and he came back home and the threat was one that we face today,” Keating said. “Through the Internet, through other linkages, that people can become radicalized and present the greatest threat.”

Keating was also part of a congressional delegation that visited Russia in May to look into the bombings. He said both trips have reinforced that if Tamerlan Tsarnaev was trying to join insurgents in Russia, he did not succeed.