St. Louis County Police arrested a 32-year-old female from Arnold for shoplifting at Walmart Thursday evening and during a search, officers found a portable meth lab in her purse. 

The Mehlville Fire Protection District and meth lab unit responded and seized the lab without incident. Walmart made the decision to temporarily close the store, but opened around 9:15 p.m., said Deputy Chief Dan Furrer with Mehlville fire. The store was closed for almost three hours. 

Portable meth labs, also known as “shake and bake” labs, are typically made out of a two-liter or 20-ounce water bottles.

The fire district responded as a precaution to police officers, since meth labs are highly flammable and explosive.

"It's a risk to the police that are handling it and it's our job to stand by and protect the police officers in case something happens," Furrer said. 

Police say theft and drug charges are pending. 

Story updated at 9:15 p.m.