What makes it impossible for Rutherford to make further research about the one who hijacked the aeroplane at Baskul? (Epilogue)

From the text:

I did. But again it was hopeless. The air force man whom the fellow had knocked out and impersonated had since been killed, so one promising line of enquiry was closed. I even wrote to a friend of mine in America who runs an aviation school, asking if he had had any Tibetan pupils lately, but his reply was prompt and disappointing. He said he couldn't differentiate Tibetans from Chinese, and he had had about fifty of the latter—all training to fight the Japs. Not much chance there, you see. But I did make one rather quaint discovery—and which I could have made just as easily without leaving London. There was a German professor at Jena about the middle of the last century who took to globe-trotting and visited Tibet in 1887. He never came back, and there was some story about him having been drowned in fording a river. His name was Friedrich Meister."