In March 2008, upon returning from a trip to Washington, D.C., I noted in this blog that I was surprised about how security seemed either lax or non-existent at some of the busiest and most popular sites in the nation's capital. I surmised that a person with cruel intentions could, with relative ease, pull off a deadly attack at the Lincoln Memorial, the Jefferson Memorial, the National Cathedral or any Metro subway station.

I didn't mention the U.S. Holocaust Memorial Museum because, unlike many other tourist attractions in Washington, security at the building's two main entrances - and even at the separate building that houses little more than the museum's cafeteria - is both visible and voluminous. Put simply, you can't get into the place without passing through a metal detector, having your bags searched and being given the once-over by an armed guard. It's a bit intrusive, to be sure, but it provides a feeling of safety.

But yesterday's shooting at the museum's 14th Street entrance - in which a security guard was killed and the 88-year-old white supremacist gunman was wounded by return fire - proves there's no such thing as complete safety. The world remains a dangerous place, and terrible people who want to harm others for no understandable reason will usually find a way to achieve their goal.

Knowing that, I guess the best we can do is keep our eyes open for suspicious behavior and hope for the best.