An airline spokesman said the Boeing 737-800 was taken out of service after the crash Monday morning.
Southwest says the ramp agent had been operating a provisioning truck, which is used to supply material for flights, including food and drink.
The plane was carrying 174 passengers and six crew members on a trip to Denver and then on to Seattle.
The airline says it hasn't received reports of any injuries among those on the plane.
An airport spokesman says its operations have not been affected.
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