The Russian parents of Tamerlan and Dzhokhar Tsarnaev, the suspects in the Boston Marathon bombing case, have agreed to visit the U.S. to assist officials in the ongoing investigation, a police source told RIA Novosti on Wednesday. A team from the U.S. Embassy in Moscow traveled to the volatile southern republic of Dagestan to speak to the Tsarnaevs, who consented to involvement in the U.S. investigation. Anzor Tsarnaev, the suspects' father, also said on Wednesday that he doubted his surviving son's ability to give testimony. "He [Dzhokhar Tsarnaev] is giving no testimony," he said. "He is even unable to speak."