Guymon said during an afternoon news conference that six cups of lye were mixed into the iced tea. He said he was shocked that the lye-based cleaning agent was kept so close to the food preparation area.
"[The investigation] is going to take a little bit of time," said South Jordan police Cpl. Sam Winkler on Thursday morning.
Police are working with the store's management to talk to not only the employees who were on shift during the incident, but the one before, as well. The management is also pulling all of the restaurant's video footage, to see if it offers any insight, Winkler said.
Harding was eating at Dickey's Barbecue Pit, 689 W. South Jordan Parkway, about 1 p.m. Sunday when she poured a glass of sweet tea from the self-service beverage bar, according to Winkler.
"She takes a sip, and her mouth instantly starts burning," Winkler said.
Harding turned to her husband, Jim, and exclaimed "I think I just drank acid!" according to a statement from Guymon.
She tried to spit out the tea, but her mouth and throat suffered severe burns, Winkler said. Her husband took her to a nearby hospital, and she was flown to University Hospital's burn unit.
The chemical is 67 percent sodium hydroxide, commonly known as lye, Winkler said. It is used to clean grease off the restaurant's fryers and appears to have been mixed with the tea accidentally.
The restaurant was cooperating with the investigation, Winkler said.
Harding, who is a retired school teacher, is intubated at the hospital and is unable to speak, Guymon said. Doctors plan on checking the damage to her esophagus and stomach, and may perform surgery on her Friday to repair any perforations, Guymon said.
Surgery on someone who is already in a weakened state like Harding's can be traumatic though, he said. Right now her family is not discussing a lawsuit, and instead focusing their energy on her, the attorney said.
Cards and encouraging phone calls have been pouring in for Harding, said Dan Walker, the executive director of the Salt Lake Baptist Association, in a statement Wednesday.
"By the grace of God the doctor on duty [when she arrived] was the Chief of the U of U Burn Unit himself. They went to work immediately and have not stopped since," Walker said.
Harding appeared more relaxed and slightly more rested by Tuesday, which has provided some rest and relief for her family, Walker said. The doctor has expressed that he is "guardedly optimistic," Walker added.
John Thomson, owner of the Dickey's Barbecue South Jordan franchise, said in a statement Thursday that he's praying for Jan Harding and cooperating with investigators. He said he would refrain from commenting on the specifics of what happened out of respect for the Hardings. His restaurant is one of 400 Dickey's around the country in the Dallas-based chain.
The Associated Press contributed to this story.