Some people are trying to push a conspiracy theory that the maintainer and the contributor are the same person.
Seems like something serious and illegal happened. Possibly to do with a hostile state. Where I live, there would be at least 2 different organisations that would be very interested to find out what happened as quickly as possible. I don't for one second think he was involved but he would be essential in any investigation.
https://tukaani.org/about.html
All meaningful new information will probably come from identifying Jia Tan, if that ever happens. I haven't done much reading yet, but it seems to me there was no connection between him and Lasse outside the Github project.
What makes you that confident they would? What if Jia actually works for them?