TOOELE, Utah -- Officials at an Army depot where nerve gas and other chemical weapons are stored found no trace of a reported intruder after a terrorist alert was sounded Thursday.

Col. Peter Cooper, commander of the Deseret Chemical Depot, said the security of the depot was never at risk and that the person didn't get close to the chemical storage area.

"At this time we cannot confirm an intruder," Cooper said. "Right now we are pretty sure we've cleared the depot. We're not sure if it was an employee who was not in the right area."

The person fled after being spotted within the heavily guarded perimeter by four soldiers during two separate patrols, Cooper said.

By late afternoon, officials were still searching the depot grounds for the possible intruder.

In Washington, a senior administration official speaking on condition of anonymity said there was no evidence that anything was stolen or that terrorism was involved.

The apparent trespasser, dressed in dark clothing, was sighted within a fenced area between the stored chemicals and the outer perimeter, authorities said.

Sheriff's deputies set up a roadblock around the depot after the alarm sounded at 9:24 a.m. and state law officers used a helicopter to search the grounds. There were no evacuations of the depot or surrounding areas.

The depot, which is about 45 miles southwest of Salt Lake City and covers 19,000 acres of mostly barren, wind-swept desert dominated by sagebrush, stores chemical weapons such as nerve gas and mustard gas.

It has been destroying a stockpile of deadly chemical weapons since 1996.

Earlier this year, it finished destroying the largest stockpile of sarin nerve gas in the United States.

It is scheduled to destroy 1,300 tons of VX, a more toxic but less volatile nerve agent, and 6,100 tons of mustard gas, a blister agent that can dissolve tissue on contact.

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On the Net

Depot: http://www.tead.army.mil

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AP correspondent Ron Fournier contributed to this report.