PUEBLO, Colo. (AP) — The Army has resumed destruction of obsolete chemical weapons in southern Colorado after a shutdown prompted by liquid hazardous waste seeping from a storage tank.

Officials say the work resumed Wednesday when another tank was returned to service. The cause of the seep is under investigation.

Pueblo Chemical Depot is destroying 780,000 shells containing 2,500 U.S. tons (2,270 metric tons) of mustard agent.

Officials say the liquid that seeped out is a byproduct of the process and contained no chemical weapons. They say less than 8 ounces (237 milliliters) escaped the tank.

Mustard blisters skin, scars eyes and inflames airways. The U.S. is destroying it under a treaty banning chemical weapons.

Since starting in 2016, the plant has eradicated 132,000 shells and 774 U.S. tons (702 metric tons) of mustard.