The two brothers suspected of the Boston bombings, Chechens who grew up in America, fit the profile of a new generation of jihadists who are radicalized online and strike in their home countries.

The motivations of Tamerlan Tsarnaev, 26, killed early Friday, and brother Dzhokhar, 19, who is wounded and in police custody, remain unclear, President Barack Obama said shortly after the second brother was captured on Friday.

A website used by Russia's North Caucasus rebels denied Sunday any link to the deadly Boston Marathon bombings that have been blamed on two ethnic-Chechen suspects.

"The command of the Vilayat Dagestan mujahedeen... declares that the Caucasus fighters are not waging any military activities against the United States of America," theKavkacenter.com website said.

Boston bombing suspect Dzhokhar Tsarnaev drove over his dying brother to escape a gunbattle with police, the police chief of the town where the manhunt ended said Saturday.

Dzhokhar and Tamerlan Tsarnaev also hurled a pressure cooker bomb, like one used in the marathon attack, at officers chasing them in the early hours of Friday, Watertown's police chief Edward Devau told CNN.

The two brothers suspected of the Boston bombings, Chechens who grew up in America, fit the profile of a new generation of jihadists who are radicalized online and strike in their home countries.

The motivations of Tamerlan Tsarnaev, 26, killed early Friday, and brother Dzhokhar, 19, who is wounded and in police custody, remain unclear, President Barack Obama said shortly after the second brother was captured on Friday.

Russia and the United States agreed Saturday to step up cooperation in their fight against terror in the wake of news that two ethnic Chechens were suspected of organizing the deadly Boston Marathon bombings.

The Kremlin said Russian leader Vladimir Putin called U.S. President Barack Obama to once again express his condolences and discuss ways the two sides can work more closely on security in the runup to the 2014 Winter Olympic Games in Sochi.

U.S. Secretary of State John Kerry on Friday refused to speculate on the implications of the reported Chechen roots of two brothers believed to have carried out the Boston bombings.

"I think it's fair to say that for this entire week we've been in pretty direct confrontation with evil," Kerry said after talks with Mexican Foreign Secretary Jose Antonio Meade.

The two suspects in the Boston Marathon bombing killed a university police officer and hurled explosives at police in a car chase and gun battle overnight that left one of them dead and his brother on the loose, authorities said Friday as thousands of officers swarmed the streets in a manhunt that all but paralyzed the Boston area.

The suspects were identified by law enforcement officials and a family member as Dzhokhar and Tamerlan Tsarnaev, brothers from a Russian region near Chechnya, which has been plagued by an Islamic insurgency that has carried out deadly bombings.

U.S. President Barack Obama vowed Thursday that the Boston marathon bombers would be found and held "accountable" as investigators focused their efforts on two potential suspects.

"Yes, we will find you, and yes, you will face justice," Obama told a special service in the city, three days after the two bombs killed three people and injured about 180 in a hail of nails and ball bearings.

FIFA chief Sepp Blatter on Wednesday condemned the deadly attacks at the Boston marathon, urging authorities to pull out all the stops in preventing such tragedies.

"What happened in Boston is a drama, a tragedy," the head of world football's governing body said in Spanish, speaking to reporters as he wrapped up a two-day visit to Cuba.

U.S. officials denied a suspect was under arrest Wednesday in the deadly Boston Marathon bombings.

A law enforcement official briefed on the investigation told The Associated Press earlier in the day that a suspect was in custody. But the FBI and the U.S. attorney's office in Boston said no arrests had been made.