A preliminary examination of cell phones and computers used by the Tsarnaev brothers finds no sign of an accomplice in the Boston Marathon bombing, NBC News reported Tuesday evening, citing “a U.S. counterterrorism source briefed on the FBI investigation.”

The source stressed that the investigation continues, but bureau officials seem increasingly confident that “nobody else was involved.”

Dzhokhar Tsarnaev, still in a Boston hospital, has told investigators he and his brother Tamerlan acted alone, learned to build the pressure-cooker bombs over the Internet and were motivated by a desire to defend Islam because of "the U.S. wars in Iraq and Afghanistan," said the source, who has received multiple briefings on the probe.

Tsarnaev has also told investigators that he and his brother got instructions on building bombs from the infamous Inspire, an online English-language magazine published by al Qaeda, federal law enforcement officials told NBC News.

The magazine has twice included articles on building bombs with kitchen pressure cookers — the method investigators say the Tsarnaevs  used in the Boston attack.