Investigators in the United States were looking into possible connections between the suspects -- brothers Tamerlan Tsarnaev, 26, and Dzhokhar Tsarnaev, 19 -- ethnic Chechens who immigrated to the United States more than 10 years ago, USA Today reported.
Tamerlan Tsarnaev, who was run over by his brother during a shootout with police, allegedly had content on his YouTube account that led investigators to believe he may have been involved with North Caucasus extremist groups, such as the Caucasus Emirate.
The older Tsarnaev also followed radical Islam, an FBI news release indicated.
However, a Russian law enforcement official told RIA Novosti the brothers did not have a direct link to any extremist groups.
Russian military expert Vladislav Shurygin Russkaya Sluzhba Novostei (Russian Radio Service) agreed: "For me, there's no clear link between [the Tsarnaevs] and the terrorist underground. But it's clear that the terror attack was connected to them wanting to play. The Internet [is] responsible. They just started browsing the Internet, found some sites connected to extremist activity, and decided to show off."
Extremist groups have also denied responsibility and condemned the attacks.
"The command of the Dagestan Vilayat states that Caucasian mujahedeen do not engage in military action with the United States of America," said a statement from the extremist Dagestan Viyalat group.
"We fight with Russia, which is responsible not only for the occupation of the Caucasus, but for heinous crimes against Muslims. Even in regards to the enemy state of Russia, with which the Imarat Kavkaz [Caucasus Emirate] does battle, there is an order in effect from the Amir of the [Imarat Kavkaz] Doku Umarov, which forbids attacks on civilian targets," the statement said.
Notable deaths of 2014 [PHOTOS]