Blackwater. A bunch of Scott Coleman type characters.
At any rate...I see no reason not to believe they are there in some capacity....who's actually hired them is another story, but just as nations have their “special forces” I think also nations use whatever means is available to affect an outcome favorable to their agenda.
In Ukraine's case there are many armed and otherwise groups in Ukraine...homegrown and foreign... hooking up together as well as operating separately...and some of these have their own personal agendas.
Then you have the media spinsters and propaganda “naming” and “blaming”.....social networks infiltrated with personnel to affect conversations.....and then there's the US/EU and Russia ‘leadership’ and politicians all chiming in and pointing the finger.
No matter how any look at this now...the dust is still in the air and the ultimate outcome questionable to say the least. All being said is basically speculation at this point.
What we definitely know is the US cannot claim “vital Interest” in Ukraine.....it's involvement is an extension of EU’s Interests and Nato involvement.....and of course McCain's personal involvement to protect his personal interests in Ukraine. Likely other politicians as well.