Soldatov said Russia's Federal Security Service, known as the FSB, uses sophisticated software to monitor social media sites, looking for incipient signs of the same radicalization process described by Hilbert.
"They might have picked up one of these guys discussing his ideas on a forum or chat monitored by the FSB and other law enforcement bodies of Russia," Soldatov said in an interview.
Investigators are also taking a closer look at the 2012 trip back to Russia for any signs the oldest brother received training or had contact with militant groups fighting since 1994 to separate from Russia, according to lawmakers briefed on the investigation.
Hilbert said that as his online interactions deepened, he was invited to visit a mosque and several religious schools, including one in the United States, which he said he couldn't name because it may still be under investigation.
The months-long grooming — beginning on public sites and continuing in invite-only web boards and forums — amounted to crowdsourcing for potential converts to a militant brand of Islam, especially ones who were already located in the U.S.
His guides were especially effective at playing on the conditions faced by recent immigrants, who often find it hard to acclimate to their new country. In the days following the attack, the suspects' uncle, Ruslan Tsarni, blamed the difficulties they had adjusting to life in the U.S. as a possible motivation for the attack, calling them "losers."
"Everything we know about the older brother makes him the perfect candidate as someone who is so unhappy in this country that the right person saying the right things will switch you," Hilbert said. "Education is the key to this process."
After he was probed on his beliefs and vetted over his claimed identity, Hilbert was eventually given passwords to private forums, where he learned how to make bombs based on models used by militants in Iraq. That access to a more trusted environment also tightened the bonds between the agent's alter ego and his guides — and all of it occurred in cyberspace.