Although Army Maj. Nidal Malik Hasan allegedly murdered 13 people in a terror rampage at Fort Hood, Texas, in 2009, this nation hasn't really experienced the sort of onslaught of politically motivated terror attacks that a number of experts predicted in the wake of 2001.

Other countries have suffered major blows from Britain and Spain to Indonesia, with multiple points in between. But for the most part, thanks often to terrific police work that has foiled literally dozens of terrorist close calls, we've been mostly immune to such events.

And thus very fortunate.

Indeed, the bloodiest mass murders in the U.S. lately, such as the one last July in Aurora, have been committed by domestic lone wolves whose motives remain either largely mysterious or bizarre.

But the Boston Marathon bombing and its extraordinary aftermath is likely to jolt us back to reality. It could well fit a more familiar mold.

To be sure, we don't yet know what motives might have propelled suspects Tamerlan and Dzhokhar Tsarnaev into the marathon bombings and later murder of a MIT police officer. However, at the very least their motives are likely to be more coherent than what we've seen, for example, from the likes of Tucson shooter Jared Lee Loughner, whose demented worldview hardly fits into any known template.

And the Tsarnaevs' purpose may be more ideological, too, given their background and what little we know of their interests.

Even if it turns out the Tsarnaevs were acting upon some idiosyncratic grudge, their actions are a chilling reminder that repeated call for vigiliance in our post-9/11 world are not an empty mantra. Moreover, the means of helping police these days can be as close as your smartphone.

It is hard to believe that terrorists with social media accounts would be oblivious to the likelihood that their actions might be tracked by cameras at a major public event. Yet it wasn't just a surveillance camera at Lord & Taylor that captured them. For example, a runner from Florida who took a picture with his iPhone caught a remarkably sharp image of the younger Tsarnaev, too.

So another lesson from Boston, it seems, is that people who instinctively turn to take photos of a tragic event aren't being busybodies. They're establishing a record that may help authorities solve a crime.

It remains true, of course, that most communities in America are extremely safe, and terrorism itself is not a new phenomenon. As The New York Times reminded readers this week, such attacks have "actually been in a steady decline since [their] peak in the 1970s."

Still, the lockdown of Boston Friday after crimes causing more than 175 casualties reminds us that the tranquility in our own communities must never be taken for granted.

The Denver Post, April 20