The source added that it should not be assumed that either brother was radicalized because of their Chechen origins.
[Updated 8:07 a.m. ET]
"All of Boston" should shelter in place, Massachusetts Gov. Deval Patrick has just told reporters. The same applies to suburbs of Watertown, Newton, Belmont, Cambridge and Waltham, he said.
By shelter in place, Patrick said he meant people should stay indoors, keep doors locked and not answer doors for anyone except for police.
Patrick also has confirmed to reporters that one Boston bombings suspect is dead and the other is on the loose.
-- A Massachusetts Bay Transportation Authority officer was "seriously wounded" and is in surgery right now.
-- An MIT security officer was killed.
[Updated 7:59 a.m. ET]
A recap of what authorities are telling Boston-area residents to do: Police ordered businesses in the suburb of Watertown and nearby communities to stay closed and told residents to stay inside and answer the door for no one but authorities.
The subway and Amtrak train systems have been shut down. Every Boston area school is closed.
"It's jarring," said CNN Belief blog writer Danielle Tumminio, who lives in Watertown.
[Updated 7:58 a.m. ET]
The Boston bombings suspect who currently is on the run has been in the United States for "at least" a couple years, a federal law enforcement source tells CNN.
[Updated 7:40 a.m. ET]
Boston police say on Twitter: "Door-to-door search 4 suspect in Watertown continues. Uniformed officers searching. Community consent critical."
[Updated 7:39 a.m. ET]
The suspects in the Boston Marathon terror attack were brothers, a terrorism expert briefed on the investigation said, according to CNN's Deborah Feyerick.
[Updated 7:34 a.m. ET]
One of two suspects in the Boston Marathon bombing -- the man police were looking for Friday morning -- has a name that is common among people from the North Caucasus, a source with knowledge of the investigation said Friday. That region includes the breakaway Russian republic of Chechnya.
Earlier Friday, The Associated Press reported that the two suspects in the Boston Marathon bombings are brothers believed to be from Chechnya.
[Updated 7:32 a.m. ET]
Police in the Boston-area community of Cambridge say the public should "clear area of Norfolk Street in Cambridge." "Ongoing investigation. Potentially dangerous," Cambridge police said on Twitter.