Short answers, pulled from the story.
The first reports of Wagner Group mercenaries in Syria appeared in late October 2015. These accounts emerged just weeks after Russia began its military intervention in the country.
Thirty-two contractors were killed and about eighty wounded during the initial push to retake Palmyra in March 2016. The group withdrew between April and May 2016 following this victory.
By July 2017, the Kremlin established a policy allowing companies that seized oil and gas wells or mines from ISIL forces to retain rights to those same sites. One company received contracts under this arrangement and was entitled to receive twenty-five percent of the proceeds from oil and gas production at any field captured and secured by its private contractors.
Russian independent media identified Stanislav Yevgenyevich Dychko, a confirmed Wagner operative formerly with the Russian Interior Ministry, as one perpetrator. A second suspect named Ruslan from Bryansk was also identified working as a patriotic educator in local schools.
American aircraft attacks resulted in the deaths of forty-five to one hundred government fighters during the clashes near Khasham on the 7th of February 2018. US defense officials assessed that Russian PMCs participated in the assault with some contractors killed in air strikes.