Duplex plugged septic drain - who's responsible?

[Rachel Kenyon]

We had a plugged septic drain in a brand new duplex (less than a year old), the plug ended up being a bunch of, what appears to be white shop towels, rags or baby wipes?  The plug was in super far so simply flushing the toilet did not back up the septic.  However, when the iron filter flushed through the water at midnight, one side had water all over their basement storage room and soaked into the carpet.  Bills will add up to almost $3,000.  Can we charge the tenant if they fess up to flushing the rags?  What if neither side of the duplex admits to it?  It is a shared septic.  Can we charge each side half?  What does everyone think?