A U.S. congressional delegation is spending a week in Russia meeting with high-level government and security officials to investigate whether more could have been done to prevent the Boston Marathon bombings.
Representative Dana Rohrabacher, the California Republican who is leading the delegation, said Wednesday: "We think there is some information that is vital for us to know that hasn't been made public yet."
Russia warned the U.S. in 2011 that bombings suspect Tamerlan Tsarnaev was a potential terrorist threat but did not respond to U.S. requests for more information.
Although the U.S. and Russian presidents have pledged to improve counter-terrorism cooperation, both sides have accused the other of being less than forthcoming.
Tsarnaev, who was killed by police, spent six months last year in Dagestan.