Apparently the asphericity is due to Zastrow (see [Cannon-Conner-Zastrow][1]). 

Also apparently the result that the higher homology groups vanish is [due to Zastrow][2], but his habilitation thesis never seems to have appeared. 


  [1]: http://www.sciencedirect.com/science/article/pii/S0166864101000050#
  [2]: http://at.yorku.ca/i/d/e/b/11.htm