They've become known as Black Hat and White Hat, two mysterious figures weaving through the Boston Marathon crowds, carrying instruments of 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 under way in Watertown, Mass. to find him.
Suspect No. 1, 26-year-old Tamerlan Tsarnaev, was killed in a gun battle with police.
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.
READ: 'Boston bombings suspects' uncle: Turn yourself in' (http://bit.ly/10nVAGX)
The brothers traveled to the United States 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.
Neighbors recalled the ethnic Chechen brothers, living on a quiet street in Cambridge, Massachusetts, riding bikes and skateboards.
Tamerlan Tsarnaev practiced martial arts and boxing, even aspiring to fight on the U.S. Olympic team. Dzhozkar Tsarnaev had been on the wrestling team at a prestigious school and won a scholarship from his city to pursue higher education.
According to a crime website, Tamerlan Tsarnaev was once arrested for domestic assault on a girlfriend.
View a timeline of the hunt on your mobile device -- http://bit.ly/Zx4KxB.
"I don't have a single American friend. I don't understand them," Tamerlan Tsarnaev was quoted as saying in a photo package that appeared in a Boston University student magazine in 2010.
He identified himself as a Muslim and said he did not drink or smoke: "God said no alcohol." He said he hoped to fight for the U.S. Olympic team and become a naturalized American. He said he was studying at Bunker Hill Community College to become an engineer.
Dzhokhar, 19, attended the prestigious Cambridge Rindge and Latin school, participating on the wrestling team. In May 2011, his senior year, he was awarded a $2,500 scholarship from the city to pursue higher education, according to a news release at the time. That scholarship was celebrated with a reception at city hall.
He attended the University of Massachusetts at Dartmouth, university officials said Friday, as the campus was evacuated. The school would not say what he was studying.
The father of the suspects, Anzor Tsaraev, told The Associated Press his younger son was a second-year medical student.
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"My son is a true angel ...," he said by telephone from the Russian city of Makhachkala. "He is such an intelligent boy. We expected him to come on holidays here."
He added, "They were set up, they were set up! I saw it on television; they killed my older son Tamerlan." He ended the call angrily, saying, "Leave me alone, my son's been killed."
Dzhokhar's page on the Russian social networking site Vkontakte says that before moving to the United States he attended School No. 1 in Makhachkala, the capital of Dagestan, a predominantly Muslim republic in Russia's North Caucasus that has become an epicenter of the Islamic insurgency that spilled over from the region of Chechnya.
On the site, he describes himself as speaking Chechen as well as English and Russian. His world view is described as "Islam" and he says his personal goal is "career and money."
On Friday, those who knew him were stunned. A host of a Boston-based public radio program, Robin Young, posted on Twitter a photo of her nephew with Dzhokhar Tsarnaev, their arms around each other, at their graduation.
"Heartbreaking pic," Young wrote.
Tim Kelleher, a wrestling coach for a Boston school that competed in 2010 against Dzhokhar's team, said the young man was a good wrestler and that he'd never heard him express any political opinions.
"He was a tough, solid kid, just quiet," Kelleher said.
Deana Beaulieu, a 20-year-old student at Bunker Hill Community College who lives two blocks away from the suspects' home, said she went to school with Dzhokhar and was friendly with his sister. She hadn't seen him since they graduated.
Beaulieu said she visited the family at their second-floor apartment in 2006. She recalled meeting the parents there.
"He was just a quiet kid," Beaulieu said of Dzhokhar. She couldn't recall his ever expressing any political views. "I thought he was going to branch off to college, and now this is what he's done. ... I don't understand what the hell happened, what set him off like this."
Dzhokhar appeared in the video released by authorities on Thursday, identified as Suspect
Number 2, striding down a sidewalk behind his brother, unnoticed by marathon spectators. He wore what appeared to be a gray hoodie under a dark jacket and pants, and a white baseball cap facing backward.
Tamerlan was wearing khaki pants, a light T-shirt, and a dark jacket. The brim of his baseball cap faced forward, and he may have been wearing sunglasses.
According to the website spotcrime.com, Tamerlan was arrested for domestic violence in July 2009, after assaulting his girlfriend. That report could not be immediately confirmed.
He was an amateur boxer, listed as a competitor in a National Golden Gloves competition in 2009. In a local news article in 2004, someone identified as Tamerlan spoke about his boxing and his views of America.
"I like the USA," Tamerlan was quoted as saying in The Sun of Lowell, Massachusetts. "America has a lot of jobs. That's something Russia doesn't have. You have a chance to make money here if you are willing to work."
The paper quoted Tamerlan's trainer, Gene McCarthy, as saying: "He has a lot of heart. That's the key." It said he loved music, and played the piano and violin.
The Associated Press contributed to this report.