Story related. Finish the first mission and kill Winkler at the end in a boss fight. It's tough because he can cloak himself and takes quite a couple of hits.
*** Spoiler - click to reveal ***when he leaves the room, follow him outside and give him another couple of rounds. When he is holding on the wing, just shoot his fingers so he lets go and dies a cruel death... i somehow did not realise it the first time. 😂