Workers installing groundwater monitoring wells at the Rocky Mountain Arsenal suspended work after air quality test detected low levels of lewisite, a chemical warfare agent.

Arsenal officials said workers at the Lime Basins Project Monday were wearing protective gear when they detected the chemical that was produced at the site in 1943.

The work is taking place in a restricted area of the arsenal, about 10 miles northeast of downtown Denver, known as a disposal area for chemical agents.

The Rocky Mountain Arsenal National Wildlife Refuge, created on parts of the site that was once a classified chemical munitions factory, remains closed.

Officials shut down the refuge after a similar discovery of lewisite on Oct. 31.