WASHINGTON—Key lawmakers said Sunday the suspected Boston Marathon bombers may not have acted entirely alone and that law-enforcement officials continue to investigate the possibility that they received outside encouragement or training.

``There are still persons of interest in the United States that the FBI would like to have conversations with,’’ said Representative Mike Rogers, a Republican from Michigan and chairman of the House Intelligence Committee, speaking on ABC’s ``This Week.’’

Many questions remain about suspect Tamerlan Tsarnaev’s 2012 six-month visit to the Russian state of Dagestan, and whether he received some sort of training from Islamist radicals while there, Rogers added.

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``There was outside affirmation of their intent to commit an act of jihad,’’ he said.

``The Russians need to step up to the plate here and provide us with information,’’ Rogers said. ``I think they have information that is would be helpful and they haven’t provided.’’

Another Republican, Homeland Security Committee chairman Mike McCaul of Texas, said some officials in the Obama administration have suggested prematurely that the brothers acted alone.

He cited the type of device used in the attack — shrapnel-packed pressure-cooker bombs. Although instructions on how to build such bombs can be found on the Internet, he said they also indicated a level of sophistication and training.

“There could be a wider conspiracy,” said McCaul, on “Fox News Sunday.” “What I found astounding is that right out of the box, U.S. officials anonymously are saying there’s no foreign connection to this case, when in fact, the FBI just began their investigation in this case.”

McCaul also said he thinks the suspects’ mother played ‘‘a very strong role’’ in her sons’ radicalization process and that if she were to return to the United States from Russia, she’d be held for questioning.

Tamerlan Tsarnaev died after a gun battle with police on April 19. His younger brother, Dzhokhar, is in custody and charged with the marathon bombings on April 15, which killed three and injured more than 260.

A number of lawmakers and counterterror specialists in the last two weeks have questioned how US authorities failed to ``connect the dots’’ on a number of warning signs and place greater scrutiny on Tamerlan Tsarnaev after Russia warned the FBI and the CIA in 2011 that he was becoming increasingly radicalized. Tsarnaev was an ethnic Chechen and citizen of Kyrgyzstan who was living legally in Cambridge after his parents won asylum in 2002. The FBI interviewed in 2011 after Russia’s warning and determined that he did not pose a threat.

Rogers said it is too early to point fingers at the FBI, although he said questions remain about whether Tsarnaev should have been interviewed a second time once he returned to the United States from Russia in 2012. Predominantly Muslim Chechen militants living in Dagestan and Chechnya have carried out terror attacks in Russia, but they have not been thought to pose a threat to the United States. At least one Chechen militant group in Russia issued a statement after the marathon bombings that said it was not involved.

Speaking on CNN’s ``State of the Union,’’ Utah Representative Jason Chaffetz asserted that the United States needs to heighten its scrutiny of immigrants as they travel.

“People that are coming here and claiming asylum, and then taking trips back to the region, that should probably raise some red flags,” he said.

Representative Jan Schakowsky, a Democrat from Illinois and member of the House Intelligence Committee, said the FBI had information that both Tamerlan and his mother were becoming radicalized. The Associated Press reported Saturday that the Russians had intercepted communications included discussion of jihad.

``The FBI thought it had more to do with internal Russian politics, and not a threat to the United States of America,’’ Schakowsky said.