/usr/lib/x86_64-linux-gnu/libfastjet.so is in libfastjet-dev 3.0.6+dfsg-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libfastjet.so.0.0.0
/usr/lib/x86_64-linux-gnu/libfastjet.so.0.0.0