According to the Associated Press, the Tsarnaev brothers were motivated by radical Islam:
BOSTON (AP) — The two brothers suspected of bombing the Boston Marathon appear to have been motivated by a radical brand of Islam but do not seem connected to any Muslim terrorist groups, U.S. officials said Monday after interrogating and charging Dzhokhar Tsarnaev with crimes that could bring the death penalty.
The brothers, ethnic Chechens from Russia who had been living in the U.S. for about a decade, practiced Islam.
