They blasted across the nation’s airwaves as the mystery men on the run from the FBI in the Boston Marathon bombings, both carrying backpacks, one wearing a backward white baseball hat and the other clad in dark glasses and a white T-shirt.

On Friday, after a chaotic night in which one of the men was killed and the other became the target of a massive manhunt, information was slowly emerging about the two men — who are brothers and the prime suspects in the attacks that killed three people and injured more than 170.

Law enforcement officials identified the suspect still on the loose as Dzhokhar A. Tsarnaev, 19, of Cambridge, Mass. His brother, Tamerlan Tsarnaev, was identified as the man killed during an encounter with police after an armed carjacking of a Mercedes SUV in Cambridge. Tsarnaev was believed to be in his mid-20s.

The brothers’ alleged motive in Monday’s bombings remains unclear, but they appear to be originally from the southern Russian republic of Chechnya, and two law enforcement officials said there is a “Chechen connection” to the bombings. Chechnya has been racked by years of war between local separatists and Russian forces and extensive organized crime since the Soviet Union dissolved in 1991. The extent of the possible connection remained unclear.

According to a database search, Tamerlan Tsarnaev was a boxer who worked out at a martial arts facility in the Cambridge area. In an Internet posting dated Nov. 2, 2011, and attributed to him by name, he wrote: “The more you know about hell, the more you want stay away from sins and keep asking Allah(s.w.t.) for forgiveness.”

The younger brother, Dzhokhar, graduated in 2011 from Cambridge Rindge and Latin School, a public high school in Cambridge. He was described as a quiet young man who was on the school wrestling team, said Deana Beaulieu, 20, who attended schools with him since the seventh grade.

She said that he lived with his parents, brother and sister at the house on Norfolk Street in Cambridge.

“He was really quiet,” she said in an interview. “They always say you have to be careful of the quiet ones.”

Dzhokhar Tsarnaev went to high school in Cambridge and worked as a lifeguard at Harvard University, a friend, Eric Machado, told CNN.

“There was no evidence that would lead any of us to believe that he would be capable of this,” Machado said in describing Tsarnaev, who typically wore his hat backward as he did in the FBI video.

The Chechen conflict dates to the early 1990s. In the summer of 1999, fighters in the predominantly Muslim republic rose up in an attempt to throw off Russian domination. Vladimir Putin, then the Russian prime minister, responded quickly, firmly and brutally to put down the rebellion.

Later that summer, there were several explosions across Russia and Putin blamed Chechens. Putin sent the army back by force, which resulted in Western criticism of Russian tactics and human rights violations.

In the most dramatic episode, about 40 armed Chechen separatists took more than 900 hostages at a Moscow theater. After a two-day siege, Russian special police pumped a chemical agent into the theater’s ventilation system and raided the building. About 130 hostages died, and all of the Chechens were killed.

Though the war has officially ended, the Russians have maintained a tight grip on Chechnya, backing a strongman friendly to Moscow and maintaining a robust military presence. Efforts have also been underway in recent years to rebuild the shattered capital of Grozny.

Still, sporadic violence and kidnapping have continued in Chechnya and separatists retain a following. The years of fighting, crime and economic difficulties led tens of thousands of Chechens to leave their homes for other former Soviet republics.

Washington Post staff writer Julie Tate contributed to this report.