According to this article, it was the second of two Su24s.
Quote:
"Turkey said its fighter pilots acted after two Russian Su-24 bombers ignored numerous warnings that they were nearing and then entering Turkish airspace. In a letter to the U.N. Security Council and Secretary-General Ban Ki-moon, Turkey said the Russian warplanes violated its airspace “to a depth of 1.36 miles and 1.15 miles ... for 17 seconds” just after 9:24 a.m.
It said one of the planes then left Turkish airspace and the other one was fired at by Turkish F-16s “in accordance with the rules of engagement” and crashed on the Syrian side of the border."
Turkey says Russian jet violated its airspace, ignored repeated warnings ? Japan Today: Japan News and Discussion