The ex-con who mowed down a 13-year-old girl in Brooklyn was a loan shark who was stabbed trying to collect a debt, Police Commissioner Ray Kelly said yesterday.

Ex-con Sean Lewis was trying to retrieve a measly $40, a police source added.

Cops said Kenneth Risher, 51, is suspected in the fatal stabbing.

Lewis, 44, smashed into Kira Goddard after she shooed neighborhood kids out of his way.