/usr/lib/x86_64-linux-gnu/libfftw3_mpi.so is in libfftw3-mpi-dev 3.3.5-3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libfftw3_mpi.so.3.5.5
/usr/lib/x86_64-linux-gnu/libfftw3_mpi.so.3.5.5