The suspects allegedly used low-grade explosives inside pressure cookers.

Investigators so far have found no evidence that the Tsarnaev brothers tested such bombs in the United States, the U.S. law enforcement official told CNN Monday.

If Tamerlan received training in making bombs, it may have come during his trip to Russia, the official said.

Sources: Russian forces kill two jihadists

Investigators are looking into whether Tsarnaev was influenced toward radicalization during a six-month visit in 2012 to Dagestan, a region known to include radical jihadists who have battled the Russian government.

Russian special forces killed two members of a jihadist group in an early morning raid this weekend in the semiautonomous republic, two Russian police sources told CNN on Monday.

Authorities have not said whether the raid was linked to the Boston bombing.

But one of those killed was an associate of Abu Dujan, the slain leader of a militant Islamist organization that produced at least one video that Tamerlan apparently posted and later removed from a social media account, according to an analysis by CNN and the SITE Intelligence Group.

'Jihad' discussed in wiretapped phone call

Russian authorities intercepted a phone call in early 2011 from one of the Tsarnaev brothers in the United States to their mother in Dagestan, an official with knowledge of the investigation told CNN over the weekend.

The wiretapped communication discussed jihad, but the conversation was vague, two U.S. officials said. It's unclear why the Russians were eavesdropping on the mother or for how long.

CNN has previously reported that the FBI conducted an investigation -- including an interview with Tamerlan Tsarnaev -- after Russia expressed concerns in 2011.

The Russians also raised questions about Tsarnaev's mother, Zubeidat Tsarnaev, according to several sources. Her name was subsequently added to a terrorism database along with her son's, an intelligence official said last week.

The FBI said at the time that it found nothing to justify further investigation and that Russia did not respond to U.S. requests for more information. The case was closed after several months.

One of the officials declined to say whether the information from the wiretapped phone call would have made a difference in uncovering plans for a future attack on the U.S.

However, CNN contributor Tom Fuentes said the FBI would have found that information helpful after Russian officials asked the agency to look at Tamerlan Tsarnaev for signs of a possible shift toward increasing Islamic extremism.

Mother vows to travel to U.S. if she can see her son

On Monday, Zubeidat Tsarnaev told CNN's Nick Paton Walsh that she plans to come to the United States if she can see her son, despite pending shoplifting charges against her in Massachusetts.

Dzhokhar is at Federal Medical Center Devens. Authorities moved him there last week from Boston's Beth Israel Deaconess Medical Center.

He had what appeared to be gunshot wounds to his head, neck, legs and hand when he was captured on April 19 after a nearly 24-hour manhunt, according to the criminal complaint accusing him in the marathon blasts.