There are a variety of factors that could come into play here. Did a pipe burst from tree roots in overly saturated ground or was there sewer backup? Was there a known history of flooding in the property? What did your home owner's insurance say and why did tenant's renter's policy deny coverage for their personal property? Have the tenants provided receipts for every gift?
Are you in a flood plain? Was the area of the basement where they placed the items an area with water marks on the walls or floor? How long have they lived there? Did you tell them when they moved in that there had been previous flooding, if you knew?
With some more information we can better assess your situation.
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My colleague brings up many valid questions which would impact any advice you'd receive from an attorney.
The long and short of it is that landlords have a legal responsibility to provide habitable housing to their tenants, and their failure to do so can result in claims against them for both the diminution of the fair market rental value of the property, as well as for actual damages to things like personal property. Further, if a lawsuit is commenced by either party, the winner can also likely be recouped by the loser for their own attorneys fees.
That, of course, is a very broad generalization of the way the law works. In application to your specific circumstances, it's unclear what liabilities you may ultimately be facing.
You should contact a landlord/tenant attorney in your area who may be able to assist you by fully analyzing the facts of the situation and determining what your liability may be.
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