SALT LAKE CITY  — A Utah military base that carries out tests to protect troops against biological attacks was locked down for nearly 12 hours overnight because a small amount of a nerve agent was unaccounted for.

It reopened early Thursday, officials said, after the missing vial was located.

The military said in a statement Thursday the amount missing was less than one fourth of a teaspoon of VX nerve agent, which affects the body's ability to carry messages through the nerves.

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Dugway Proving Grounds spokeswoman Paula Thomas said the base, located about 85 miles southwest of Salt Lake City, had first reopened to incoming personnel, and people inside were allowed to leave shortly thereafter.

She said there were no injuries resulting from the cause of the lockdown. About 1,200 to 1,400 people — a mix of military personnel and contractors and civilian workers — were inside the base when the lockdown occurred, Thomas said.

The Salt Lake Tribune reported the nearly 800,000-acre base conducts chemical and biological defense training, and "is the Defense Department's leader in testing battlefield smokes and obscurants."

Personnel there also test military equipment's viability in environments where they're facing chemical or biological threat.

The base also is used by the U.S. Army Reserves and the U.S. National Guard for maneuver training.