For the second time in recent days, a teen-ager minding his own business has been shot dead in Washington.

Monday's Washington Post reported that the latest victim was 16-year-old Donnell Plater. He was walking home after doing some Christmas shopping when he was accosted by three youths intent on robbing him. When Donnell tried to evade them, he was shot in the back.

Washington's homicide squad has solved many cases of this kind and has brought many killers to what passes for justice these days. However I have for a long time been of the opinion that our judicial system ignores one important aspect of these cases.

We ought to put great emphasis on finding out who sold a handgun to the person who pulled the trigger. If there was anything illegal about the transaction, the supplier of the weapon ought to be prosecuted as an accessory to the crime.

In the present case, if the gun was not stolen it was very likely bought in the black market that flourishes "on the street." When a black market sale is made in Washington, a person not licensed to deal in guns sells one in a jurisdiction in which permits to buy guns are strictly regulated, and a jurisdiction in which it would be almost impossible for a juvenile to buy or own a gun legally.

If we made it a practice to prosecute illegal gun sellers as accessories to the crimes committed with the guns they supply, I think the word would get around pretty quickly that there are more prudent ways to make money than to deal in black market guns.