/usr/lib/x86_64-linux-gnu/libnfft3f_threads.so is in libnfft3-dev 3.3.2-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libnfft3f_threads.so.2.0.0
/usr/lib/x86_64-linux-gnu/libnfft3f_threads.so.2.0.0