Key removed engine still running slightly

This symptom just started, I turn the key to the off position and remove it. The dash lights go off but the engine continues to run, but sounds like it's power is diminished. Reinserting the key and turning it on, the engine speed increases and runs at a normal idle.

If it is left off with the key removed, it will die out after about 10 seconds. Haven't seen this before. Any ideas for the cause?