FLEXTRA was downloaded from:  http://flexpart.eu/
where the following License was found:

  Copyright 1998-2010 Andreas Stohl <ast@nilu.no>

  Authors:
  Andreas Stohl, Petra Seibert, A. Frank, Gerhard Wotawa, 
  Caroline Forster, Sabine Eckhardt, John Burkhart, Harald Sodemann 

   FLEXPART is free software: you can redistribute it and/or modify 
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or 
   (at your option) any later version. 

   FLEXPART is distributed in the hope that it will be useful, 
   but WITHOUT ANY WARRANTY; without even the implied warranty of 
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 
   GNU General Public License for more details. 

On Debian systems, the GNU GPLv3 is available at /usr/share/common-licenses/GPL-3


FLEXTRA is mature software; FLEXPART is still undergoing development, and the files
in FLEXTRA can be found in FLEXPART with explicit GNU licenses attached.
( Flextra is labelled open-source in the website; in the later revisions of the
files in Flexpart, explicit GNU GPL declarations have been added.)