Which level of government has the power to pass ex post facto laws?

An insurer issued a homeowner’s policy that covered collectible historical documents. the documents, valued at and insured for $70,000, were lost in a fire that destroyed the home. prior to paying the homeowner, the insurer received a demand from a third party that the insurer pay the $70,000 to the third party. the third party contended that the documents had been stolen from the third party. the insurer and third party were citizens of the same state, while the homeowner was a citizen of a different state. the insurer would like to file a federal statutory interpleader action in the federal district court for the state of which the homeowner is a citizen. although this state has a long-arm statute, neither that statute nor the general service of process rules would permit service of process on the third party. the insurer does not wish to deposit the $70,000 with the court or post a bond pending resolution of this matter. of the following, which is a valid reason for advising against the filing of a federal statutory interpleader action in the federal district court? adiversity of citizenship does not exist between the insurer and the third party.