Nothing is harder for cops than the death of one of their own from another cop's bullet. So you have to feel for the BART police, the family of slain Officer Tom Smith Jr. and for the man who shot him.

What we know so far -- and it is precious little -- is that a single gunshot passed through a gap in Smith's bulletproof vest and struck him fatally in the chest, an extraordinarily uncommon outcome.

I don't pretend to know all the answers about the tragedy inside a Dublin apartment last Tuesday. But as we think about avoiding such accidents in the future, here are some of the questions:

A) Just how did the shooter hold his gun? Police typically are trained to point their weapons down, with their fingers outside the trigger guard. Once they perceive a threat, they raise the gun. In this case, investigators say they entered with guns drawn.

We don't know where they were pointing their pistols. But in this case, was it possible that the cop's finger was on the trigger?

B) Did the issue of "trigger pull" enter into this case? In the old days, when police used revolvers, it could take 10 pounds of pressure or more to pull the trigger. But with modern semiautomatics like the Glock or Sig Sauer pistols that BART police use, it can take only 4 or 5 pounds. Experts say it's extraordinarily unlikely that the gun would accidentally discharge. But in the adrenaline of the moment, could the shooter have misjudged how little pressure was needed to fire a pistol?

C) Could the shooter have mistaken Smith for an intruder? This is always possible. But we simply don't know enough to conclude that. Ordinarily, when cops enter a residence, they are taught to "peel off" toward the sides, minimizing the chance that they will shoot one another.

The key questions here are the trajectory of the fatal bullet, and where Smith was standing when he was hit. Investigators have released nothing yet on those issues.

D) Should the BART police be doing this kind of operation in the first place? This question goes directly to the training that BART cops receive. It's not just a matter of police academy lessons. The truth is that BART cops don't get as much chance to do home searches as regular city cops. That increases the possibility that something can go wrong.

In the Oscar Grant case five years ago, the lack of training given to BART cop Johannes Mehserle was painfully evident. He said he mistook his gun for his Taser.

BART announced with great fanfare that it had made reforms after the Grant shooting -- and Tommy Smith was one of the officers responsible. But those reforms were focused on how the force dealt with the public on transit property. They did not really prepare the cops for a home search that could go haywire.

In the end, making a case against a burglary suspect isn't worth the loss of an officer's life. If searches like the one in Dublin have to continue, it argues for a more elite team, perhaps in concert with other agencies. Interestingly enough, an Alameda County sheriff's deputy accompanied BART cops in Dublin.

Contact Scott Herhold at 408-275-0917 or sherhold@mercurynews.com. Follow him at Twitter.com/scottherhold.