The answer is when it is home-grown.
The mainstream media largely ignored a story about an especially sophisticated and deadly backpack bomb found along a Martin Luther King Day parade route in Spokane, Washington last week, barely covering it beyond an initial mention. The device drew special attention from some news outlets because it contained shrapnel, was equipped with a remotely-controlled detonator, was "directional" (meaning aimed toward the parade route) and in the FBI's words, was "capable of inflicting multiple casualties." The major media barely mentioned the incident, and the lack of follow-up stories on it is even more deafening now that the FBI has concluded that the connection between this incident and racism is "inescapable."
The mainstream media has been loathe to address the bigger story around this bomb, which is the rise in right wing extremism occurring right here at home in the U.S.
Terrorism Is not Just from Foreign Sources
To most Americans, terrorism is something that comes solely from abroad. The early explanation fed to Americans for continuing the war in Iraq was that we had to "fight the terrorists over there, so we don't have to fight them here." The idea that American citizens are generating deadly terrorist activity within our own borders is something that many Americans, like the mainstream media, would rather not acknowledge, let alone discuss.
The attempted bombing of Times Square by a man with a middle-eastern-sounding name last year resulted in blanket coverage for days followed by weeks of speculation by the mainstream media, while home-grown, right wing extremism and terrorist activity gets lower-grade coverage and fades from the headlines much more quickly.
Recruitment and Radicalization
In 2009 the Department of Homeland Security issued a report warning law enforcement officials about the rise in "rightwing extremist activity" in the U.S. Using language usually reserved for a discussion of Muslim terrorists, the report explained that the economic recession, the election of the first African-American president and the return of disgruntled war veterans was fueling recruitment and radicalization of extremist groups within the U.S. The report also defined extremist groups as not only racist and hate groups, but also as groups hostile towards federal authority, like anti-abortion and anti-immigration extremists.
The Southern Poverty Law Center, which tracks hate groups and domestic terrorists, reports that 15 different hate groups now operate inside Washington state alone, with six more just over the border in Idaho. These are white power militias and neo-Nazi groups with names like the "Aryan Terror Brigade" and "Aryan Nations. "But race is just one part of the multifaceted problem of domestic American extremism. American religious, anti-government and anti-abortion extremists have shown they are willing to commit murder to further their cause, and now there is a growing wave of anti-immigration extremism.
The first step in defusing the growing problem home-grown, right-wing terrorism is making the population aware of the existence, scale and severity of the problem. That can't happen until the mainstream media gives at least equal weight to coverage of domestic terrorism incidents, so they are on par with the coverage foreign terrorism incidents.