There's a Chinese short story in which a young man lies in bed, paralyzed from a bicycle accident. With nothing else to do, he mentally unwinds the events that led him to such a pass, working backward from the accident.

Each step seems insignificant in itself. This happened on the heels of that, and so forth, going all the way back to, incredibly enough, a rude sound emitted by a dog on the street. That sound, he concludes, launched the other events in motion and ultimately caused his personal tragedy.

The reader can't help thinking what the author probably intended: Why stop there? Who's to say previous events didn't lead to the encounter with the dog, and so on back in time to the cooling of the Earth's crust?

That's the thing about cause and effect in human events. No matter how far you unspool the reel, there's always some other factor in the background. So it's tempting to say that if only X, Y or Z hadn't happened, the outcome would have been different.

Such is our need to feel some control over our lives. We want to believe that things happen logically, for a reason, and that when they go wrong, we can pinpoint the crucial moment that explains everything.

The entire city of Pittsburgh has been agonizing over this for the past week, ever since a conspiracy wingnut with an arsenal gunned down three Pittsburgh police officers in cold blood on a residential street in Stanton Heights.

Once again we are forced to consider the nation's self-destructive insistence that all its residents, including whack jobs with grudges, are entitled to more firepower than the police who are sworn to protect the rest of us from them. Until that changes, no amount of analysis will stop the carnage.

Still, we pour over the train wreck of that morning when Richard Poplawski, 22, killed Pittsburgh police Officers Paul J. Sciullo II, Eric Kelly and Stephen Mayhle, looking for something that makes sense. As the stories in this newspaper unfolded day by day, more bits of information kept coming out.

The shooter's mother woke him up at 7 a.m., upset that the dog had peed on the floor. In the ensuing argument, the gunman's mother called 911 to remove her son, telling the operator that there were legal weapons on the premises. But the 911 operator did not pass the weapons information on to the police dispatcher.

The shooter later told police that his mother saw her son strapping on his body armor but still opened the door for the officers to enter. In any case, the officers walked into an ambush. The shooter's bullets ripped through their bullet-proof vests, but the police bullets did not penetrate the shooter's body armor. So while three cops -- husbands, fathers, brothers and sons -- were summarily executed, the shooter was wounded only in the legs.

The gunman's friends called him a good guy, bright and nonviolent -- even though he had a penchant for paranoia-fueling, racist and anti-Semitic Web sites; even though he'd attacked his ex-girlfriend and been kicked out of Marine Corps boot camp for assaulting an officer; even though he believed that the "Zionist-controlled" Obama administration would come to take away his guns.

The day after the killings, the shooter's grandmother accused police of stealing her wallet from the purse she'd left in the house. The shooter's father, who hadn't seen his son for more than two years, said he had no idea what his son was into.

This scenario offers ample junctures for "what if." If the mother hadn't thought an argument over dog pee warranted a call to police. If the 911 operator had conveyed a warning about weapons. If the mother had warned the police about her son's guns when she opened the door to let them in. If his friends hadn't made excuses. If his family life had been somehow different.

In the end, though, none of these factors gets the blame for the chaos that broke out in Stanton Heights that morning. The blame rests solely with the misfit who immersed himself in hate fantasies and armed himself to the teeth, waiting for them to come true. When he wrapped himself in armor, opened fire on three officers, walked up to two of the downed men and shot them again just to make sure they were dead, those were the defining moments.

This gunman was his own cause and effect.

That probably won't make the emergency operator who committed such a grave human error feel any better, and Allegheny County 911 is right to implement a new protocol for handling information about weapons.

But of all the things that happened that day, only this makes sense. Officers Sciullo, Kelly and Mayhle answered the call to duty, and even as they lay riddled with bullets, their fellow officers tried to save them, putting their own lives on the line.

We can replay that day's chain of events a thousand times, but the place to stop will always be the same. The officers fulfilled their sworn duty with honor and selflessness. The shooter played his part like the paranoid killer he had been preparing to become, enabled by easy access to firepower that could drop an elephant in its tracks. In the search for explanations, there is no need to look any further than that.