Lease Obligations Questions!!!!

Arizona
This is an odd problem for me. However, I was reviewing my credit report a few days ago and noticed a deliquent account regarding money owed to a realty company in Tucson. After reviewing the details it appears to be from an apartment I rented back in 2004 with a now ex-boyfriend. We rented an apartment in September of 04 and by the end of December, we broke up and I gave a thrity day notice of vacating the residence. The only lease terms upon signing was that it was month to month from the beginning. I vacated the residency by the end of January and the ex-boyfriend remained in the unit until May of 05. Now I have a collection agency calling me and letting me know that I am still responsible for the damage he caused prior to moving out in May because I was never taken off the lease. What I am confused about is that I moved out and why would I be held responsible for what he did especially since the lease terms were month to month? Any help or explainations would be great! Thanks
Furthermore, the new renting company that took over the complex in March sued the ex in May of 05, which I was not named a party of nor summoned to court

If the landlord approved your move out simply write the agencies and inform them--have the LL agree and sign if need be

Well that's another interesting ordeal, the landlord was fired and soon after passed away. the collection agency refuses to send me any paper work regarding the unit. the other thing is that in may of 05 the ex was taken to court by the new leasing agency for the damages and i was not named a party or as part of the lease then after reviewing the case he went to court for it looks as tho they sued him for the damages without me included and why would i suddenly be named a party afterwards? secondly the court system had my name and address because around that same time i was involved in suit against my old landlord for security deposit issues which i won. i understand that there is a possibility that i am responsible for this and i just wanted to pose those questions before agreeing on paying any amount of money.

[able1]

You can certainly dispute it and make them prove you owe...they may give up.