They've become known as Black Hat and White Hat, two mysterious figures weaving through the Boston Marathon crowds, carrying instruments death. One suspect is dead, one still at large. But who are they?
According to several law enforcement officials and the suspects' uncle, the two men are brothers.
The Associated Press identified suspect No. 2, in the white cap, as 19-year-old Dzhokhar A. Tsarnaev of Cambridge, Mass. A massive manhunt is underway in Watertown, Mass., to find him.
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Suspect No. 1, reportedly 20 years old, was killed in a gun battle with police. NBC News identified him as Tamerlan Tsarnaev, Dzhokhar Tsarnaev's older brother.
Their uncle, Ruslan Tsarni of Montgomery Village, Md., told The Associated Press that the men lived together near Boston and have been in the United States for about a decade. They traveled here together from the Russian region near Chechnya.
Russia's North Caucasus region has been plagued by an Islamic insurgency stemming from separatist wars in Chechnya.
The Associated Press contributed to this report.