https://en.wikipedia.org/wiki/2016_Berlin_attack
The first significant conspiracy theory claim comes from long-time false flag theory writer Stephen Lendman, and repeated by Iran's state run media site PressTV
http://www.presstv.ir/Detail/2016/12/20/500727/Germany-Market-Attack-CIA-US-Lendman
PressTV is generally very critical of the US, and has entertained many conspiracy theorists in the past, including Veterans Today who claim that the Sandy Hook school shootings were a hoax.
http://www.presstv.ir/Detail/2016/12/20/500727/Germany-Market-Attack-CIA-US-Lendman