Spy numbers station on shortwave

I've heard this strong station several times now over the past year, usually later at night. It always broadcasts at 5900 kHz (or now that I think about it, have I picked it up at 9800kHz?). I picked this up Monday night/Tuesday morning, April 21st, just after 00:34 CDT (5:34 UT).

The station is pretty strong here in Chicago, with only occasional fading, and it sends a modified CW signal that can be picked up with any shortwave receiver. It only uses three digit code letters in groups of five: those Morse Code letters that are described by three combinations of dots and dashes, minus S and O, which are dot-dot-dot and dash-dash-dash.

5900-spystation.mp3 audio file, 7.43MB

Decoded, the file yields this:
DAUWN WIWNW GDNWR UTWAT RGAID INWTA NUGIW UIWWA GINRT NRDWI DNRUT RDAUG NNAII TWIIR UDDNA ATDGT NUART DTIWU IUWWI TINUW ADNIT RRRWT DATRA DUWGW NGINI UTDTR IDIRA DRUDD NIDTU IIDRG URWUU GTTNI UIWIR UITND ATTDI WARGR DGTGD DWIRD AIIAI ARDTI TIGDU GRUWI AUTRT NTWNU GRUUU TUGUG AAARI TRRNG TTDAA WUWGU NNATW WNGRN TNRAN DRUTN DWUGU RDAUN RGGNN TNWNN WDGDD WARGA DDGNN DUDTA RRNNN RWTNU RUWTI GRTIA UDAAN ITIUG UIIAR WIUTU IATNT UDNGN ARGNT TAAWU WUADN DAARW INRGR GNNAG ADGRU GNWWR GWGUD WDNNA URRAR WITIN RRDAD AIDRR GUAUG INANU DIWUI WAGAN NRUUN AWRAD DWRNR UTRAU INIDR UITUN ATDUG RUUAA UIGWT DAIUI AIRRD ANAIN IDATI WDIUG WTNAD UNRAI GNDTR ADNNI DUNGR RDWDI UGTUD TDATG GIGUG IAANA DRDTD TGIGN GWUNN GWIII ADWDD

The actual contents of this message are presumbly only known by the intended spy and the agency that sent the one-time pad message to them.