The US Army has located a small amount of a deadly nerve weapon that went missing at a Utah military research base, officials have said.
Some 1,200 to 1,400 people were locked in the 1,200 sq-mile Dugway Proving Ground for several hours after a vial containing a quarter of a teaspoon of VX nerve agent was reported missing.
Officials said no-one was hurt and no damage reported.
Base officials said the nerve agent had been found early on Thursday.
Officials planned a news conference on Thursday afternoon.
The primary mission for the Dugway Proving Ground, which is located about 85 miles (135km) south-west of Salt Lake City, is defending troops against biological and chemical attacks.
The vial of nerve agent was discovered to be missing during a routine check of sensitive materials.
"As a precaution, the commander immediately locked down the installation and began efforts to identify the cause of the discrepancy," the US Army said in a statement.
The vial was located in the laboratory several hours later.
"All personnel are uninjured and safe. The public is safe as well," the statement said.
Scientists at the base, which is roughly the size of the US state of Rhode Island, test military equipment against exposure to biological and chemical attack, in order to learn how best to protect US troops on the battlefield.
The post is among the most isolated spots in the US - the nearest town of any size is about 40 miles away across a mountain range.
The nearest major city is Salt Lake City, home to the Mormon church's headquarters and site of the 2002 Winter Olympics.
VX nerve agent was developed in the 1950s and is chemically related to insecticide. A fatal dose can kill within 15 minutes. The weapon attacks the nervous system, causing uncontrollable convulsions, loss of consciousness, paralysis and death.
It was never used in combat by the US, the US Army has said.