Why do violent national tragedies so often spark eruptions of irresponsible speculation among liberal journalists seeking to link the latest horror to their political opponents on the Right? This corrosive pattern was especially evident in the hours following the Boston Marathon Bombing. Even before the identities of the bombers were known, for example, NPR's Dina Temple-Raston announced that "April is a big month for anti-government, and right wing, individuals. There's the Columbine anniversary. There's Hitler's birthday. There's the Oklahoma City bombing. There's the assault on the Branch Davidian compound in Waco. And the FBI right now is comparing this to the Eric Rudolph case [the 1996 Olympics bomber]."

Temple-Raston is a veteran counter-terrorism beat reporter, a former Bloomberg White House correspondent, and author of multiple books and articles on national security and civil liberties issues, so she especially ought to know the pitfalls of such irresponsible speculation. Unfortunately, similar examples of political blame-shifting have become all too common in recent years. Shortly after the July 2012 Aurora theater massacre in Colorado, ABC investigative reporter Brian Ross embarrassed himself and his network by trying to link the shooter to the Tea Party based solely on a shared name on a web site.

Before that, the New York Times editorial board argued that "it is legitimate to hold Republicans and particularly their most virulent supporters in the media responsible" for the January 2011 shooting of Rep. Gabby Giffords and the murder of six of her constituents. In both the Colorado and Arizona killings, what little was then known about the two individuals behind the shootings suggested they were of doubtful mental stability, but that didn't prevent Ross and the Times editorial board from trying to link the heinous crimes to Republicans and Tea Partiers.

But it's not just Republicans and Tea Partiers who are injured. As The Washington Examiner's Philip Klein explained in a Friday Beltway Confidential post, ideologically driven speculation is no substitute for credible, fact-based reporting: "The reason why conservatives get irked when 'right wing' is used in reference to major acts of violence -- often without an iota of evidence to back it up -- is that the term 'right wing' is broadly applied by the media to the entire conservative movement. I don't think 'right-wing' Jennifer Rubin [Washington Post blogger] and Sheldon Adelson [Las Vegas gambling magnate] get pumped every April for Hitler's birthday, that 'right-wing think tanks' like the Heritage Foundation burst out the champagne on the Columbine anniversary, or that 'right-wing rock star' Scott Walker is a big fan of the Oklahoma City bombing."

What is now known about Dzhokhar Tsarnaev, the 19-year-old surviving brother of Tamerlan Tsarnaev, 26, is that these young Chechen men came to America a decade ago and appeared to have become comfortable members of suburban Boston communities. There are indications of radical Islamic influences in their lives, but it remains to be established whether those influences were of sufficient importance to prompt the Tsarnaev brothers to commit mass murder. Only fact-driven investigation can supply the answer to that crucial question.