The United States has suffered yet another bloody massacre of its citizens — by one of its own. The immediate reaction to a bloodbath in which 12 people were killed and 58 wounded was shock and horror at the scale and senselessness of the tragedy.

But the extraordinary truth is that this happens too often to be considered just another "tragedy." It was mass murder facilitated by complicity, if not cowardice. The ease with which Americans can get access to high-powered weapons that serve as instruments of slaughter is impossible for any non-American to comprehend.

Mr. James Eagan Holmes was a quiet, unassuming medical student with a pretty impressive record but little else to distinguish him — until July 20. On the evening of that day in Aurora, Colorado, he bought a ticket for an early screening for "The Dark Knight Rises," the latest Batman movie.