Most epic disasters have a silver lining - unifying pride swelled by the heroic actions of emergency responders, law enforcement authorities and, often, ordinary citizens - that transcends so much collective horror and anguish.
After Sept. 11, Americans rallied together to show the world their spirits could not be broken.
Similarly, the wake of Hurricane Katrina brought a national outpouring of support for the stricken Gulf.
Newtown and the nation were embraced as never before following the Sandy Hook Elementary School massacre.
And last week's bombings at the Boston Marathon brought out the best qualities in a community at first wracked by bloody terror, then paralyzed by fear and, finally, overcome by grateful jubilation when police apprehended a suspect.
Sooner or later, though, it seems that such sweetness and light inevitably and depressingly revert to bitter divisiveness.
It did not take long for finger-pointing to replace group hugs after 9/11 and Katrina; Sandy Hook quickly provoked rancorous arguments and heated political debate over gun control.
Now, just days after the capture of Dzhokhar A. Tsarnaev, 19, and the death of his 26-year-old brother, Tamerlan, we have gone from scenes of joyous Boston residents dancing in the streets and even hard-hearted New York Yankee fans tenderly singing "Sweet Caroline," the theme song of their hated rival Boston Red Sox, to angry exchanges in barrooms, the media and the halls of Congress over immigration, religion and due process.
The brothers Tsarnaev, born in Russia's troubled Chechnyan region, immigrated with their family about a decade ago, moved to Massachusetts and were granted refugee status. Tamerlan was a permanent resident and held a Russian passport; Dzhokhar became a naturalized U.S. citizen on Sept. 11, 2012.
Their status has reignited arguments over U.S. immigration laws, and the older brother's reportedly slavish devotion to radical Islam has thrown more fuel onto already inflamed relations between Muslims and followers of Judeo-Christian religions.
In addition, the government's decision to begin questioning Dzhokhar without first reading him the Miranda warning of his right to remain silent and have a lawyer present has rekindled contentious debate over how the criminal justice system should handle terrorism cases.
One camp insists those accused of terrorism should be treated as enemy combatants and not be given the Miranda warning; the other side argues that all suspects deserve such protection - especially an American citizen being interrogated on U.S. soil.
The Obama administration is relying on a 1984 Supreme Court ruling that decreed suspects don't have to be "Miranda-ized" if authorities feared an immediate threat to the public.
It was based on the case of an accused rapist, said to be armed. When police caught the suspect and spotted his empty holster, they asked, "Where's the gun?" before reading the Miranda rights, and his lawyer later attempted, unsuccessfully, to have the charges dropped.
This newspaper has high regard for the extraordinary local, state and federal authorities who tracked down and eventually apprehended Mr. Tsarnaev, and support continuing law enforcement efforts to determine if the brothers were part of a larger terror cell plotting to wreak additional mayhem.
At the same time we object to twisting the 1984 high court ruling, thereby trampling a judicial protection all citizens deserve.
While we're on the subject of positive and negative aspects of contemporary law enforcement, we were heartened by how quickly the public helped police by supplying photographs and descriptions of the bombing suspects and, in the end, by phoning in a report that led to the younger brother's capture.
But it was also disturbing how quickly some social media sites triggered what amounted to a vigilante mob suspicious of every foreign-looking passerby.
Whatever our feelings about the investigation, above all, we mourn the victims, and must do what is right to honor their memory.