American diplomats arrived in the Russian Caucasus to interview the parents of Dzhokhar and Tamerlan Tsarnaev, suspected of bombing the Boston Marathon, the US Embassy in Moscow said on Wednesday, DPA reported.
In Boston, the street where the bombings occurred reopened to the public and thousands gathered for a memorial service for slain Massachusetts Institute of Technology police officer Sean Collier, who was killed Thursday at the start of a day-long manhunt that ended in the death of Tamerlan Tsarnaev and the capture of Dzhokhar Tsarnaev.
Vice President Joe Biden told the mourners that the brothers who allegedly planted the bombs at the marathon's crowded finish line were "twisted, perverted, cowardly, knockoff jihadis."
Dzhokhar Tsarnaev meanwhile remains in fair condition in a Boston hospital, the FBI said. He has reportedly been communicating with investigators, claiming the pair acted on their own without involvement from an outside group. The mission by embassy officials to republic of Dagestan is part of a joint effort by Russia and the United States to solve the April 15 terrorist attack, which killed three people and injured more than 200. Officials had at first said 176 people were wounded, but the figure rose as more people sought treatment for minor ailments in the days after the attack.
"The FBI is receiving cooperation from the Russian government in its investigation of the Boston Marathon bombing," the embassy said in a statement.
The Tsarnaev brothers lived in Makhachkala, the capital of Dagestan, before their family moved to the US in 2002 or 2003. Their parents, Anzor and Zubeidat, later returned to the city situated on the western shore of the Caspian Sea.
The mother told human rights workers that US officials were "very polite" during the interview, according to the Interfax news agency.
Investigators have focused on the predominantly Muslim region after it emerged that Tamerlan Tsarnaev, the elder of the brothers, who died after a shootout with police on April 19, may have spent up to six months there in 2012.
Dagestan has for years been a hotbed of radical Islamists and terrorists, leading to speculation that Tamerlan was inspired by extremists battling local authorities.
The White House declined to comment Wednesday on the status of the investigation or whether the men had connections to foreign terrorist groups.
"I don't think we have all the answers yet, and we won't for some time. That's why we need a comprehensive investigation," spokesman Jay Carney said.
He noted that US officials have long been concerned about the threat of "self-radicalization" and said national security officials had discussed the threat.
Secretary of State John Kerry, referring to Tamerlan Tsarnaev's trip to Chechnya, said, "He learned something where he went and he came back with a willingness to kill people."
State Department spokesman Patrick Ventrell however later stressed that Kerry was not implying "any more specific information about this case" or implying that Tamerlan Tsarnaev had been radicalized in Russia, but was speaking to radicalism more generally.
Russian officials denied any terrorist links between Dagestan and the Boston bombings on Wednesday.
Regional Interior Minister Abdurashid Magomedov told Interfax that while Tamerlan was in Dagestan to renew his passport last year, there was no evidence of connections to terrorists.
According to the ministry, Tamerlan Tsarnaev had no contacts with North Caucasus rebels during his stay, Magomedov said. He added that Tsarnaev's father, Anzor, said that his son spent only three or four days in Dagestan.
US Secretary of Homeland Security Janet Napolitano told a congressional hearing Tuesday that the elder Tsarnaev brother's trip raised red flags.
The FBI said it investigated Tamerlan Tsarnaev at the request of a foreign government but had turned up no terrorist connections.
President Vladimir Putin had offered Russian assistance to investigate the bombings as early as April 16.
When the suspects' identity was established three days later, Putin and US President Barack Obama agreed during a telephone conversation to cooperate on the case.