▁highly ▁effective ▁intelligence ▁network . ▁They ▁had ▁been ▁aware ▁of ▁the ▁presence ▁of ▁the ▁ 5 0 0 th ▁SS ▁Par ach ute ▁Battalion ▁in ▁Y ug oslav ia ▁for ▁some ▁time , ▁and ▁of ▁the ▁general ▁threat ▁of ▁an ▁air bor ne ▁assault ▁for ▁over ▁six ▁months . ▁They ▁may ▁have ▁become ▁aware ▁of ▁the ▁isol ation ▁of ▁the ▁ 5 0 0 th ▁SS ▁Par ach ute ▁Battalion ▁or ▁the ▁concentration ▁of ▁transport ▁aircraft ▁and ▁gl iders ▁at ▁Zag reb ▁and ▁Ban ja ▁Lu ka ▁over ▁a ▁month ▁before ▁the ▁operation . ▁The ▁Part is ans ▁also ▁managed ▁to ▁re capt ure ▁the ▁deser ter ▁Sk or zen y ▁had ▁inter rog ated . ▁As ▁a ▁result ▁of ▁these ▁early ▁indic ators ▁of ▁an ▁attack , ▁T ito ' s ▁main ▁headquarters ▁was ▁re located ▁to ▁another ▁cave ▁near ▁the ▁village ▁of ▁Bast asi , ▁ ▁west ▁of ▁Dr var . ▁T ito ▁then ▁used ▁the ▁Dr var ▁cave ▁during ▁the ▁day , ▁but ▁returned ▁to ▁the ▁Bast asi ▁cave ▁at ▁night . ▁In ▁addition ▁to ▁this ▁pre ca ution , ▁elements ▁of ▁the ▁ 6 th ▁L ika ▁Pro let arian ▁Division ▁were ▁moved ▁closer ▁to ▁Dr var . ▁ ▁On ▁ 2 3 ▁May ▁ 1 9 4 4 , ▁a ▁single ▁German ▁F ies eler ▁F i ▁ 1 5 6 ▁reconna issance ▁aircraft ▁fle w ▁a ▁number ▁of ▁parallel ▁runs ▁up ▁and ▁down ▁the ▁Una ▁valley ▁over ▁Dr var ▁at ▁around ▁; ▁activity ▁consistent ▁with ▁conduct ing