One child died and three others were sickened by E. coli after coming into contact with animals at the San Diego County Fair, California health officials said Friday.

A 2-year-old boy was hospitalized and died Monday from a complication, the San Diego County Health and Human Services Agency said in a statement.

The other three children did not require hospitalization. They all visited the fair between June 8 and 15 and showed symptoms from June 10 to 16, the agency said. The four cases involved children 2 to 13 years old. Their names were not released.

Four cases of E.coli bacteria linked to San Diego County Fair. pic.twitter.com/0zRZTNgU5l — SD HHSA (@SDCountyHHSA) June 29, 2019

While the source of the E. coli is under investigation, "all children had a report of visiting the animal areas or the petting zoo, or had other animal contact at the San Diego Fair," the health department said.

Fair officials have closed public access to all animal areas, including the petting zoo. More than 1 million people have visited the fair, which will run until Thursday.

"Our hearts, our prayers, our thoughts go out to the family and the friends of this young child," Timothy Fennell, CEO of the Del Mar Fairgrounds and general manager of the county fair, said at a Friday night news conference. "We're devastated."

Fennell said they were notified of three E. coli cases Friday morning and learned that a child died that evening, and acted immediately to follow health department guidance. He said that it appears the children were involved with contact with possibly livestock animals or the petting zoo.

Fennell said the fair would continue. "No contamination has to do with food service or food at all. That’s been verified,” he said.

The Department of Environmental Health re-inspected food facilities visited by the children and found no link to the cases, the health department said.

There are many strains of E. coli, but the children were sickened with Shiga toxin-producing E. coli. People are infected by swallowing tiny — usually invisible — amounts of human or animal feces, according to the Centers for Disease Control and Prevention.

"Our sympathies go out to the family of the child that died from this illness,” county public health officer Wilma J. Wooten said in a statement. "While most people recover from this illness without complications, 5 to 10 percent of people diagnosed with STEC develop the life-threatening kidney infection."

The fair opened May 31. On average, 1.4 million to 1.6 million people visit each year, and so far this year, more than 1 million people are estimated to have attended, a fair spokesman said.

Symptoms of infections from Shiga toxin-producing E. coli can vary, but often include diarrhea, severe abdominal cramps and vomiting, according to the health department.

The CDC estimates that Shiga toxin-producing E. coli causes around 3,600 hospitalizations and 30 deaths each year in the United States. The highest risk for severe illness is children younger than 5, adults older than 65 and those with weakened immune systems.

The county health department said the most important step in preventing infection is good handwashing hygiene, and it reminds people to always wash their hands thoroughly after contact with animals or their environments.