What is the right strategy regarding the killer who claims that someone else ordered the murder? Where does this murderer's word stand according to Islam? Should we believe him or not? And what if the one who supposedly ordered the murder denies ever even having met the murderer?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

The issue mentioned in the question must be referred to an Islamic court to study all its details, as it is more appropriate that a court looks into such matters.

Allah knows best.