Cousin of Accused Bomber Tamerlan Tsarnaev Probed as Source of Radicalization

May 9, 2013 — -- Investigators have widened their search into possible contacts between one of the alleged Boston bombers and extremist militants in Russia's turbulent Caucasus region. This time, they are looking into whether the suspect, Tamerlan Tsarnaev, was influenced by a distant cousin who is said to have ties to extremist Islamist groups in Dagestan.

The cousin, Magomed Kartashov, is said to be the founder and leader of a group called The Union of the Just, which reportedly promotes the application of Islamic Sharia law and has protested against the United States.

A lawyer for Kartashov confirms that Russian security agents recently interviewed her client about his links to Tsarnaev.

The possible ties were first reported by TIME Magazine, which noted that while The Union of the Just publicly renounces violence, several of its members have ties to militants

The lawyer, Patimat Abdullaeva, confirmed to ABC News that her client met with Tsarnaev last year, but denied that Kartashov was an extremist. Instead she says it was Tsarnaev whose views were extreme. She said the two met several times because they are family, but denied claims by Tsarnaev's mother to TIME that the two had a close relationship.

Kartashov is currently in jail on charges of resisting police in an unrelated matter. His lawyer expects he will remain there for at least two more months.

Tamerlan Tsarnaev, 26, was killed during a violent standoff with police on April 19, days after he and his brother Dzhokhar, 19, allegedly planted homemade bombs at the finish line of the Boston Marathon. Dzhokhar Tsarnaev was captured alive and has been charged with using and conspiring to use a weapon of mass destruction. He faces the death penalty.

Investigators continue to burrow into Tamerlan Tsarnaev's six month visit to Dagestan, a restive region in southern Russia that is home to an Islamist militant insurgency. They want to know if and how Tamerlan attempted to join militant groups there.

Earlier, a U.S. official confirmed to ABC News that investigators are also looking into Tamerlan's contacts with a young militant named Mahmud Mansur Nidal. The two were reportedly seen leaving a controversial Salafist mosque in the capital of Makhachkala that has been popular with militants over the years. They also want to know about possible contacts with a Canadian-Russian militant named William Plonikov, who was killed in a police shootout last July, just days before Tamerlan suddenly left the country.

American officials have praised recent cooperation with Russia in the investigation.

FBI Director Robert Mueller visited Moscow for talks on Tuesday that were described as "productive." The same day, Secretary of State John Kerry arrived in the Russian capital for meetings with President Vladimir Putin and Foreign Minister Sergei Lavrov. Their discussions centered on resolving the conflict in Syria, but they also discussed the investigation into the Boston bombing.

All of that followed two calls between Putin and President Obama since the attack took place, during which the leaders pledged to continue counter-terrorism cooperation, according to White House statements.

The relationship has not always been so rosy.

American and Russian law enforcement and intelligence services still regard each other with suspicion, a holdover from the Cold War. While each side has provided the other about terror tips, including a 2011 request to look into Tamerlan Tsarnaev's intentions to join extremist groups in Russia, U.S. officials have said that Russia only provided them all of the information about their concerns about him after the April 15 attack took place.