Not very good English, but this is what Al Jazeera have to say:
Russia steps up Idlib strikes after Sukhoi jet downing | Russia News | Al Jazeera
The pilot of the Sukhoi was killed after being ejected from the aircraft by rebel fighters, including members of the Hay'et Tahrir al-Sham group, commonly known as Tahrir al-Sham. The group is spearheaded by the former
al-Nusra Front, which used to be
al-Qaeda's branch in Syria.
Analyst Richard Weitz of the Hudson Institute told Al Jazeera it was too early to say how exactly the plane was brought down. "Some reports say it was brought down by machine-gun fire. There's a couple of reports out there saying it was shot down by a missile but rebels are not known to have surface-to-air missiles with this kind of range," he said.