A northeastern Forsyth County supermarket has been deemed safe, though authorities are at a loss to explain what sickened several people and forced the store's evacuation Sunday afternoon.

Four people were hospitalized and the Publix on Keith Bridge Road at Hammond's Crossing was cleared for nearly five hours after what officials first thought was an ammonia leak.

Forsyth County Fire Capt. Jason Shivers said the department's hazardous materials team was summoned and 24 people went through a decontamination process that included taking a shower in a mobile unit and changing into fresh clothes.

Shivers described it as "fairly localized event," affecting people in just one section of the supermarket. The store reopened about 7:30 p.m.

Publix spokeswoman Brenda Reid said Tuesday morning it was still not clear what caused people to become sick.

"The fire department came in and tested the air," she said. "They did not find anything out of the ordinary that would have caused that kind of reaction.

"They're cognizant of the situation, and if anything happens in the store, we have a process in place where our associates can alert us."

Reid said the affected area was close to the deli.

"So the food that was in the deli - the food that was exposed to air - was disposed of," she said. "At this time the fire department has cleared [the store] and declared the air to be safe."

Shivers said no mechanical failures were found inside the building.

Publix maintenance is looking into the situation, he said, and the store's security department is checking surveillance footage to see if someone may have released a chemical into the air.

Reid said the company was "cautious on an ongoing basis," but emphasized that the fire department "did not find any suspects in the store to indicate the air was not safe."

The incident happened about 2:45 p.m. Sunday. Shivers said fire and emergency medical personnel received reports of a woman complaining of vomiting and difficulty breathing.

"We were afraid at first that there could be some sort of anhydrous ammonia as part of the cooling system in the building, but our technicians arrived and performed a very thorough evaluation and that structure is not cooled with any form of ammonia," he said.

Shivers said that other people who tried to help the woman complained of similar symptoms. The woman and three others, whose names have not been released, were taken to Northside Hospital-Forsyth for treatment. A fifth person was treated at the scene.

A hospital spokesman said Monday the four people had been released.

A similar incident happened Tuesday morning at the Wal-Mart Supercenter on Windward Parkway and Hwy. 9, just south of Forsyth County.

The store was closed about 7 a.m. after four employees complained of lightheadedness and difficulty breathing.

Mark Stephens, battalion chief with the Milton Fire Department, said the incident is "just a coincidence at this point" with the north Forsyth situation.

"As luck would have it, one of the hazmat techs working [Tuesday] also works for Forsyth County, and he ended up being on both incidents, but he said the odor was completely different," Stephens said. "So we're pretty confident we had two separate unrelated events."

Allie Taylor, a spokeswoman for the city of Milton, said a Wal-Mart employee reported a sulfur-like smell coming from the store, which managers then evacuated.

The four employees were taken to North Fulton Regional Hospital for evaluation. Other shoppers and employees were checked out in the parking lot.

Alpharetta's hazardous materials team searched the facility twice, but was not able to determine a cause.

"They originally thought it was a chemical spill, but they no longer think it's true, based on the readings on their devices," Taylor said.

She said the Alpharetta crew handed the investigation back to Wal-Mart, which hired a private company to continue looking into the incident.

Online Editor Jim Dean and Staff Writer Jennifer Sami contributed to this report.