Unmasking the assailant and the kidnapper of Grace van Pelt.

The assailant was Donny Culpepper, the man kidnapping Grace van Pelt was Dr. Linus Wagner.
Why?

Because
on the press release both actors were listed as guest-starring(as Culpepper and Wagner), but
didn't appear at all throughout the entire episode, except in the
picture of the suspect list.

You might argue that the picture appearance on the list
alone was sufficient for a credit as guest starring. If that was the
case why were the actors playing Haibach, Volker, Cooper, Ed Hunt and
Buddy Hennings not credited for their picture appearance?

The kidnapper was a rather small and thin guy while the assailant appeared much more muscular. Culpepper has a pretty solid stature, so out of those two only Wagner fits the frame of the kidnapper, therefore Culpepper must be the assailant...