Whether its insurance (as in third party), or as in the case of the Russian diplomat recently found dead, where police are not looking for a 'third party' why are they not looking for a 'second' party? If one person is involved in something and they believe someone else may be involved, surely they are the 'second party' and not the 'third'?
Is it because the 2nd party would be you? They are looking for he or she; From grammar
1st person I or We
2nd person You
3rd person He, She, They or It
With insurance, the first and second parties (in either order) are the insured person and the insurer. Hence anyone else involved in a claim is a third party.
With a police enquiry about a death, the police and the victim are the first two parties in the enquiry (since they are both automatically involved in it). Anyone else (such as a suspect) is a third party.
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