That would be Olin, which makes the Winchester brand of ammunition, and owns the trademark, which is only licensed to US Repeating Arms, when it was spun off from Olin.
Actually Olin is Headquartered in Clayton, MO. The Winchester Ammunition division is indeed headquartered in East Alton, IL. But that has no longer has anything to do with Winchester brand firearms, which have been built by US Repeating Arms since 1981. USRA is currently owned by Herstal, the parent company of FN and Browning.
Actually the three companies have a long history of cooperation. FN made John Browning designs in europe for many decades, Winchester made many other Browning designs in the US. The Winchester-Browning relationship goes back to the time of John Moses Browning himself.