There is a special sadness in the aftermath of Virginia Tech’s observance of a day of mourning last week. Classes were canceled, and ceremonies were held to honor the 32 students and faculty members killed just a year ago in the deadliest school shooting in American history. It was fitting and proper to pay respect.

There is a note of irony, however, in the fact that we have no foolproof way of preventing future tragedies of this nature, even on the “aware” Blacksburg, Va., campus. People are murdered in offices, in fast-food restaurants, in churches and public schools and in other types of public gatherings. We can never be sure that we are safe in today’s turbulent, unpredictable climate where there seems to be increasing frequency of troubled and disturbed individuals striking out at what they consider to be hostile entities.

Granted, there have been efforts to “get help” for individuals who seem inclined to violence. There are new security procedures and systems that are designed to deal with emergencies. Yet for the most part these steps are taken after the fact; they may lessen the thrust of assailants but they don’t anticipate and prevent.

On April 16, 2007, a student from Fairfax County with a history of mental health troubles struck at a dormitory and academic hall. Seung Hui Cho shot and killed 32 people, wounding at least 30 and traumatizing countless others before killing himself. The 2008 campus observances were intended to help the healing process but some felt the anniversary “ripped open a lot of wounds for a lot of people” and perhaps heightened their fear of new trouble.

There is every reason for to reflect and evaluate how to recognize dangerous people and get them out of society, but it is a difficult puzzle to solve. Such events have been going on for decades, with the University of Texas tower rampage by sniper Charles Whitman in 1966, one of the first such events. He killed 14 and wounded 31 before being shot by law enforcement personnel.

We didn’t learn enough then to prevent tragedies such as Colorado’s Columbine High and an even bloodier Virginia Tech assault. Even though we may think we have something of a handle on such events, the fact is we don’t and perhaps never will.

The sorriest aspect of the Virginia Tech memorial services is that they serve only to remind us how we go through life trusting people to do the right thing while increasingly we encounter erosions and vicious violations of that trust. There are no hiding places anymore, anywhere.