(CNN)A Florida woman is facing charges after she went to a Walmart in Tampa, got items from the shelves and started making an explosive device inside the store, authorities said.
Then the woman, Emily Stallard, 37, tried to detonate it in the store Saturday, the Hillsborough County Sheriff's Office said in a statement Tuesday.
"A security guard with Walmart noticed the woman ... roaming the aisles of the store and opening unpaid items. The items included flammable materials, projectiles and matches," the sheriff's office said.
The security guard called the sheriff's office and also notified an off-duty Florida Fish and Wildlife Conservation officer who was in the store at the time. The officer and the security guard stopped her just before she lit a wick and detained her until deputies arrived, CNN affiliate WFLA reported.
Deputies rushed to the scene when they got reports of a customer trying to ignite a bomb inside the store.
"This woman had all the supplies she needed to cause mass destruction at her disposal," Sheriff Chad Chronister said.
"Had it not been for an alert off-duty law enforcement officer and a watchful security staff at Walmart, she may have followed through with her plans to cause an explosion inside the store."
Chronister urged people to report any unusual activities, saying it could help avert a tragedy.
"I can't stress enough: if you see something, say something. You don't have to tackle a bad guy to be a hero. One phone call to law enforcement when you spot something or someone suspicious can ultimately save lives," he said.
Stallard had a child with her at the time of the incident, authorities said. She was arrested on charges of attempted arson of a structure, fire bombing, contributing to the delinquency of a minor, child abuse and battery on a law enforcement officer. Deputies said she spit on them while she was being arrested.
She was held on $8,500 bond, according to the affiliate. Information on her attorney was not immediately available.