Officials say they’ve reached a milestone at a Kentucky chemical weapons depot with the destruction of all projectiles containing mustard agent.

The last projectiles containing mustard agent were destroyed Saturday at the Blue Grass Chemical Agent-Destruction Pilot Plant in Madison County, the facility said in a statement this week.

The U.S. is destroying its chemical weapons stockpile under an international treaty. Mustard and nerve agent are being destroyed at the Blue Grass Army Depot in Kentucky while mustard agent is being destroyed at a Colorado facility.

The chemical weapons stockpile at the Kentucky site originally contained 523 tons of chemical agent configured in 155mm projectiles containing mustard and VX nerve agent, 8-inch projectiles containing GB nerve agent, and M55 rockets containing GB and VX nerve agent.

The last of Kentucky’s projectiles containing VX nerve agent were destroyed in May. Now that the mustard agent projectiles are gone, only rockets are left to destroy.