Army specialists plan in the next few days to re-enter the building in the Edgewood Area of Aberdeen Proving Ground where a mustard agent leak was detected last week to start a thorough search for the cause.

Officials said they expect to complete their risk assessment by midweek and then enter the sealed building and drape industrial plastic sheets over the containers that hold the mustard agent, a deadly chemical once used in weapons.

"We will then place monitors under the sheets," said Mary Jo Civis, Edgewood chemical activity civilian executive, in a statement. "Once we are able to detect the vapor from a specific stack, we will narrow our search and individually wrap and monitor each container in that stack."

A trace amount of mustard vapor was detected Wednesday in a storage building at Chemical Agent Storage Yard. No vapor has been detected outside the storage building, and no further vapor has been detected within the structure, Army officials reported yesterday.

Civis said the new procedure will take time to complete, given the number of containers in the storage structure.

"There have been instances at other stockpile sites where a very small amount of vapor has leaked from a container [that] then sealed itself and stopped leaking," Civis said. "In such cases, we are dealing with extremely small levels of agent vapor, typically well below exposure limits set by the Occupational Safety and Health Administration."

Once investigators have identified the container that caused the chemical alarm to sound, technicians will seal the leak and take the container to the nearby Aberdeen Chemical Agent Disposal Facility, where the mustard agent will be drained and neutralized. The resulting byproduct is mostly water and will be tested to ensure that the agent has been destroyed, Army officials said in a statement.

Emergency personnel responded to a chemical alarm at the yard about 9 a.m. Wednesday. The building in which the alarm sounded was sealed, and a portable chemical agent filter was attached to its ventilation system to prevent any release of vapor outside the structure.

The mustard vapor leak was the first recorded release from the stored containers in the 60-year history of the chemical stockpile at the Edgewood Area, the Army statement said.

Since beginning disposal operations in April last year, workers have safely neutralized more than 300 tons of mustard agent, officials said.