Beach concluded that evidence of German intentions had been collected by air reconnaissance, spy reports and debriefings of refugees and escaped prisoners of war but that German deception measures made information gleaned from intermittent air reconnaissance during the frequent bad flying weather over the winter appear unremarkable. German digging behind existing fortifications had taken place several times during the Somme battle and led British Intelligence to interpret the evidence of fortification-building further back from the Somme front, as an extension of the construction already being watched. In late December 1916, reports from witnesses led to British and French air reconnaissance further to the south and in mid-January 1917 British intelligence concluded that a new line was being built from Arras to Laon. By February, the line was known to be near completion and by 25 February, the local withdrawals on the Fifth Army front and prisoner interrogations, led the Anglo-French to anticipate a gradual German withdrawal to the new line.