Am I responsible for paying utilities after I have terminated my lease? If not, who is?

I rented an apartment in a complex for a year and a month without problems. Water was included in the rent but electric was separate. More than a year after leaving, I received a notice from a collections agency for an unpaid bill from the power company. The bill was for the month after I had left the apartment, confirmed by asking the apartment manager and checking when I paid for moving. When I turned in my key I specifically asked if there was anything I had to do with regard to the power company (I had to call and put it in my name at the lease's start) and was informed that I was "all set." I've heard that utilities return to lessor's responsibility when a lease is terminated. I have excellent credit, never late or unpaid, and the amount is small, but I don't think I'm liable.