Boston bombing investigators continue to probe the idea that Tamerlan Tsarnaev knew or was influenced by someone in the Islamic militant community in Russia. Unfortunately, all the potential suspects who could talk about it happen to be dead.

CNN is reporting on Tuesday that federal agents are looking at Tsarnaev's connection to William Plotnikov, a Russian-born Canadian who returned to his home country to join the jihadist movements there. Like Talmerlan Tsarnaev, Plotnikov was an aspiring boxer who came to North America as a young teenager, and he was also in Degestan in first half of 2012. Plotnikov was killed by Russian security forces in July, just days before Tsarnaev returned to the United States.

According to a report in the Russian newspaper Novaya Gazeta, Tsarnaev also met with yet another militant, a 19-year-old named Makhmud Mansur Nidal, during his trip to Dagestan last year. Nidal was also killed in a gun battle last spring after authorities accused him of helping to plot a bomb attack in Dagestan's capital, Makhachkala. That same fate befell Shakhrudin Askhabov and Gadzhimurad Dolgatov, two other alleged militants from Dagestan that were also killed by Russian forces within the last year and have been tangentially linked to the Boston attackers.

Authorities in America still haven't been able to prove a connection between Tsarnaev and the numerous militants who might have crossed his path, either online or in Russia. But as we said before, everyone who could confirm or deny these reports has already been killed by Russian soliders. The Novaya Gazeta report does claim that Plotnikov gave Tsarnaev's name to Russian agents who were interrogating him about his online contacts, suggesting that the two men communicated online, or were at least aware of each other through social media. Beyond that, however, nothing else is known for certain about their relationship—including whether Plotnikov's death prompted Tsarneav to leave Russia earlier than planned or drove him take up the fight on behalf of the fallen militants.