Runaway Cummins

Yesterday morning, I started the 1995 Pete 379 I have been diving for last three years. Cummins engine (sorry, don't know model). All normal until I started feathering throttle to get air up. Barely hit pedal and the engine instantly went to WOT. Turned key off and NOTHING HAPPENED - ending kept running flat-out. This was totally new to me and in fact I've never even heard of it. RPMs too high to put in gear and kill it that way. Another driver saw what was happening and ran over. I showed him the key, out of ignition. The truck has a kill switch and he hit that. RPMs dropped slightly and motor started running very rough. Finally Rs dropped low enough I could get it in gear and kill engine. Whole time it was running, other guys said thick black smoke pouring out of the stacks like a locomotive.
Boss arrived about then. He restarted the truck (kill switch off) and it did not go wide open, but ran pretty badly, again with lots of smoke coming out.
He theorized a seal might have broken in turbo causing it to pump oil in to intake, which sort of made sense.
Anyone else out there ever experience this, and if so what caused it?
FWIW, I've been driving for about 8 years now and still consider myself a newbie. I learn something new every day and would sure like to put this one in perspective. BTW, I didn't need any coffee to wake up that day!

 

Appreciate the replies. Engine is tan in color. I’m still thinking turbo since the kill switch cuts fuel off (doesn’t it?) - which I think would mean it couldn’t be injectors or fuel related at all ... right? FWIW, this is a replacement used engine of unknown mileage that up until now has run fine. We swapped it for the previous one that DID put a rod through the block, but not for same reason.