Russian authorities are revealing that the Boston bombing suspects' mother, who has been proclaiming her sons' innocence, was caught on wiretap discussing jihad with her older son, Tamerlan. NBC's Jeff Rossen reports.

Russian authorities have told U.S. investigators they recorded a 2011 telephone conversation between Boston bombing suspect Tamerlan Tsarnaev and his mother in which the pair discussed jihad, according to a source briefed on the investigation.

The conversation, which the Russians have said was captured electronically via a wiretap, was not presented to U.S. authorities until after the April 15 bombing that killed three people and injured 264 near the finish line of the Boston Marathon, according to the source.

Tamerlan Tsarnaev also mentioned the possibility of a trip to Palestine during the conversation with mom Zubeidat Tsarnaeva, the source said.

The phone call does not indicate there was clear intelligence that the now-deceased 26-year-old was plotting an attack at the time, counterrorism experts said.

The revelation comes as investigators are looking into questions about whether Tamerlan Tsarnaev and his brother, Dzhohkar, had any help or training to execute the attack. A key focus of the investigation centers on what Tamerlan Tsarnaev did during a 2012 trip to Dagestan, southern Russia.

Several former counterterrorism officials told NBC News that investigators are pointing to major mistakes the brothers made as evidence the duo was untrained and worked alone. Those mistakes include the fact that they made themselves stand out by calmly walking away from the site of the explosions as those around them panicked. They also had no escape plan and returned home after the bombing. They also allegedly later had to carjack a man's SUV and steal money using his ATM card.

Tsarnaev died in an early-morning shootout with police on April 19. His 19-year-old brother was taken into custody later that evening.

A badly wounded Dzhokhar Tsarnaev now resides in a federal Bureau of Prisons medical facility in the town of Ayer, 40 miles west of Boston. He has been answering some questions from investigators.

NBC News' Michelle Franzen contributed to this report.

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