One piece of intel the player finds notes that the most successful attempt to create a Veil Assassin used a captured and Brainwashed resistance fighter as the test subject... and suggests that all future Veil Assassins (including, presumably, the ones you have to fight) be created in this way.
When B.J. first meets the Kriege brothers, one of them name-drops the Manhattan Project to flaunt the black market's intelligence sources. Blazkowicz is said to be visibly shocked upon hearing the code word. While it's technically possible they could have at least learned of the code name, why on earth would it have any meaning for B.J., who not only has no connection to the project, but as someone whose duties involved a high risk of capture would be among the least likely to be briefed on any such secrets?
In a later encounter with the Kriege brothers, they appear to have obtained a complete dossier on B.J., name-dropping previous missions such as Operation Resurrection and the Spear of Destiny... and yet they say they've never heard of the Office of Secret Actions before.