How did Black Mesa do their NPC animations?

I’ve been modding Half-Life 2 recently and have been working on adding custom animations to existing npcs. I’ve made some good progress so far, but I’m having issues adding animations to models with gestures and movement animations. I know some of Black Mesa’s animations are from Half-Life 2 (for example: the scientist’s gestures and most of the soldier animations), and I’m curious what their work flow was or what hoops they had to jump through.

I know they added a bunch of zombie slump animations as well as some dynamic interaction stuff, but did they have to make new attack and walk animations? I can’t seem to remember. This would be extremely helpful, since one of the problem npcs I’m dealing with are zombies.

Founded in 2004, became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.