Nor it just the Post—"infiltrator" is also used by the Israeli news site Ynet and by the Religious Zionist Arutz Sheva.
One, that he might possibly have been an infiltrator working for the FBI?
infiltrate in·fil·trate (ĭn-fĭl'trāt', ĭn'fĭl-)
v. in·fil·trat·ed, in·fil·trat·ing, in·fil·trates
To cause a liquid to permeate a substance by passing through its interstices or pores.
To permeate a porous substance with a liquid or gas.