Mustang Start problem

Recently I turned her over and the starter kept engaging with the key on the on position. I quickly turned off the car and took the key out but the starter was still turning, I disconnected the battery cables to make it stop. Just went back out there to see if maybe it had stopped or something came unstuck, connected the battery back up and its still doing it. Only I forgot the car was in reverse ;-) that was a fun five seconds of trying to take the cables back off haha, but my question is what could be causing this? And where would be the first place to start lookin. Thanks

I would suspect something is shorting your starter. Check the connections on your starter solenoid that none of the wires are overlapping touching other terminals and that everything is dry. Disconnect the negative from the battery first because the positive from the battery goes to your solenoid and you don't want it to be hot when your working on it.
What year is your mustang and how many miles are on it?

Its an 87 GT, about 41k on the motor and accessories