"We cannot begin to comprehend how this horrible tragedy occurred," the family of Tsarnaev's wife Katherine Russell, 24, said in a statement released to PEOPLE Friday.
"Our daughter has lost her husband today, the father of her child. In the aftermath of the Patriot's Day horror, we know that we never really knew Tamerlane [sic] Tsarnaev," they said of their son-in-law, 26.
"Our hearts are sickened by the knowledge of the horror he has inflicted. Please respect our family's privacy in this difficult time."
Russell converted to Islam for her husband, according to the U.K.'s Daily Mail. The couple have a three-year-old daughter, Zahara.
On Friday night, Tamerlan's brother, Dzhokhar Tsarnaev, was taken into custody by police.
Monday's bombings near the finish line of the marathon left three people dead and 176 injured.