SALT LAKE CITY (AP) — Jan Harding and her husband had just arrived at a Utah restaurant for a relaxing lunch with friends when she filled her cup with sweet tea from a self-serve beverage station.

The 67-year-old grandmother took one sip before spitting it out and exclaiming to her husband: “I think I just drank acid.”

In fact, the tea was laced with a highly toxic industrial cleaning solution meant for degreasing deep fryers, authorities said. It contained the odorless chemical lye, the active ingredient in drain cleaners.

Four days later, Harding was in critical condition Thursday at a Salt Lake City hospital’s burn unit, unable to talk and fighting for her life, lawyer Paxton Guymon said. She hasn’t improved since Sunday, when she and her husband went to Dickey’s Barbecue Pit in South Jordan after church.

Investigators and the restaurant manager have told the Hardings that a worker mistook the cleaning product for sugar and accidentally mixed large quantities of it into the iced-tea dispenser, Guymon said.

“It’s disturbing that this kind of toxic, poisonous material would be in the food prep area and somehow find its way into the iced tea vat,” the attorney said. “I don’t know how something like that can happen.”

South Jordan police are still investigating how the chemical ended up in the sweet-tea container, but they think it was accidental, Police Cpl. Sam Winkler said. South Jordan is a suburb of 60,000 about 15 miles south of Salt Lake City.

Investigators are reviewing video footage from inside the restaurant and interviewing staff who worked that day and in the days leading up to the incident.

They have determined Harding is the only victim, Winkler said. It appears she was the first to drink the tea that day, and restaurant employees dumped it out after she was burned, he said.

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.

Guymon said he will wait for the police investigation to finish before determining what legal action to take.