FOR ALL the unknowns about Monday’s mass shooting at the Washington Navy Yard, there was a nauseating familiarity to the unfolding events: the witness accounts of chaos and fear; the plea from officials for the public’s help; the number of dead revised upward and then revised upward again.

“We still don’t know all the facts, but we do know that several people have been shot and some have been killed. So we are confronting another mass shooting, and today it happened on a military installation in our nation’s capital,” said President Obama. Another mass shooting. Again, again, again.

This time, at least 12 people were killed when, for unknown reasons — as if there ever could be reason — a gunman opened fire on workers hurrying to start a new week at the venerable military complex in Southeast Washington. About eight other people, including at least one police officer, were injured in the 8:15 a.m. incident in Building 197, which houses the Naval Sea Systems Command. Once again, Americans silently extended sympathy to the families of those so senselessly killed. Once again, we wordlessly dispatched prayers and hopes of recovery to the wounded.

A 34-year-old former Navy reservist identified as the shooter, Aaron Alexis, was also killed. Two other men were initially identified as potential suspects, but D.C. Police Chief Cathy L. Lanier said Monday night that investigators were confident that Mr. Alexis was the only gunman.

The investigation, involving multiple agencies, was being led by the Federal Bureau of Investigation. More was uncertain than certain as this page went to press Monday night. Was this an act of terrorism fueled by a political agenda? Retaliation by a disgruntled employee? Were others aware — or should they have been — of the gunman’s intent? How did he acquire his weapons (an AR-15 assault rifle, a shotgun and a semiautomatic pistol were reportedly found on him), and how did he bring them onto a gated facility? What was to be learned about security procedures and the police response?

The shootings — the deadliest such incident ever in the District — interfered with many lives: Schools were locked down; jets were temporarily grounded at Reagan National Airport; the Senate adjourned early; nearby roads were closed; the Washington Nationals postponed their night game; loved ones waited anxiously for word. But for most Washingtonians, for most Americans, life went on — the life of a horrified bystander.

Life does go on, through Columbine in 1999, through Virginia Tech in 2007, through Sandy Hook in 2012. Each atrocity provides a jolt to the nation and then recedes with little effect, until the next unimaginable event occurs, except each time a little more imaginable. Everything was supposed to change after a man with a semiautomatic weapon mowed down 20 elementary school children in their classrooms last December. But for the politicians, nothing changed. Now, another massacre, another roster of funerals. Again, again, again.