Mourning when the body is not buried and hope of burial is lost

A person kidnapped in Gaza, who is confirmed (according to testimonies and recordings) to have been murdered there, and the body is still held in Gaza. Should one sit shiva before the burial?

Since he was kidnapped not for money, as soon as hope of burying him is lost, all the laws of mourning apply, even if they lost hope thirty days after the death.

Shulchan Aruch, Yoreh De'ah, Siman 370, Paragraph 5: "Victims of the government who are not allowed to be buried, from when do they begin mourning and counting seven and thirty days? From the moment they lose hope of asking the king to bury them, even if they have not lost hope of stealing them." The same applies to all bandits who have kidnapped a body and do not want money but are involved in matters of life and death; the laws of mourning apply to them. And even if they lost hope after thirty days, it is not considered distant news, as the Shach writes there, s.k. 6, and the reason is that since the body is not buried and it is known that he died, and they tried to bury him, the count of days does not start from thirty days, as usually mourning begins from the burial, not from the death.