/usr/include/fastjet/internal/ is in libfastjet-dev 3.0.6+dfsg-1.1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/fastjet/internal/BasicRandom.hh
/usr/include/fastjet/internal/ClosestPair2D.hh
/usr/include/fastjet/internal/ClosestPair2DBase.hh
/usr/include/fastjet/internal/Dnn2piCylinder.hh
/usr/include/fastjet/internal/Dnn3piCylinder.hh
/usr/include/fastjet/internal/Dnn4piCylinder.hh
/usr/include/fastjet/internal/DnnPlane.hh
/usr/include/fastjet/internal/DynamicNearestNeighbours.hh
/usr/include/fastjet/internal/IsBase.hh
/usr/include/fastjet/internal/LimitedWarning.hh
/usr/include/fastjet/internal/MinHeap.hh
/usr/include/fastjet/internal/SearchTree.hh
/usr/include/fastjet/internal/Triangulation.hh
/usr/include/fastjet/internal/Voronoi.hh
/usr/include/fastjet/internal/base.hh
/usr/include/fastjet/internal/numconsts.hh