NEWPORT, Ind. -- Army officials said Wednesday a drain plug that dislodged in a chemical reactor caused about 300 gallons of caustic wastewater to spill at a plant built to a destroy western Indiana's stockpile of a deadly nerve agent.

Tuesday's spill at the Newport Chemical Depot halted chemical neutralization of the VX nerve agent until the problem is corrected. It was the fourth such spill since May 2005, when an Army contractor began destroying VX at the complex.

Lt. Col. Scott D. Kimmell, the depot's commander, said in a statement Wednesday that a preliminary investigation shows two employees were trying to tighten a drain plug at the bottom of one of the depot's two chemical reactors when it became dislodged.

That allowed a wastewater called hydrolysate, which is the byproduct of the neutralization process, to spill onto the sealed concrete floor and into a sump designed to contain liquids.

Kimmell said the Army will continue investigating until it determines the "root cause" of the spill of the hydrolysate, which the Army has compared to liquid drain cleaner.

No one was injured or exposed to the hydrolysate, which Kimmell said contained no active VX, which is so deadly that a single droplet can kill a human within minutes.

A federal review continues into the Army's plan to ship the hydrolysate to a DuPont Co. plant in New Jersey for final treatment and disposal into the Delaware River.

The Army reported that as of Tuesday about 33,375 gallons of VX had been destroyed -- about 13 percent of the more than 250,000 gallons originally stored at the depot about 30 miles north of Terre Haute.

The total cost of the project now stands at $1.2 billion.