All four children were hospitalized, said Dr. Alan Melnick, the health officer for Clark County, Wash.
Health officials learned of the first hospitalization involving the E. coli O157:H7 bacteria strain on March 19, he said. The other three children were hospitalized soon afterward and have since been released.
Melnick wouldn't provide any further details on the child who died.
"We believe we have it confined," he said Friday.
On April 2, county public health officials temporarily closed the home-based, licensed day care center operated by Dianne and Larry Fletch in Vancouver, near Portland, Ore.
The Fletch Family Daycare has operated since 1990 and no complaints have been filed against it, Winter said. It has about 22 child attendees, Melnick told reporters.
"This is a very difficult time for the family who has suffered such an incredible loss," the Fletches said in a statement Friday. "It is also a difficult time for our day care families and the children who were his friends. It is an especially difficult time for us as day care providers."
The statement said the day care has worked closely with the county health department to put measures in place to control the spread of the illness.
The Fletches did not immediately return phone messages from The Associated Press seeking additional comment.
E. coli is a common and ordinarily harmless bacteria, but certain strains can cause abdominal cramps, fever, bloody diarrhea, kidney failure, blindness, paralysis and death.
The strain involved in this case, E. coli O157:H7, is best known for its role in large outbreaks traced to ground beef or produce.
However, person-to-person transmission can be a problem in day-care settings or nursing homes without sufficiently thorough hand washing after toilet use or diaper changing. In some cases, especially in young children, infection can lead to life-threatening complications.
The day care is housed in a tidy single-story yellow rambler on a large lot set back from the road and behind a gated driveway. A 6-foot-tall chain link fence encloses the property.
Investigators have not pinpointed a precise source of the outbreak, but Melnick said the infection was spread from person to person.
"When we did our investigation, there was not a 'point' source," he told The Columbian newspaper in Vancouver. "Not all of them got sick at the same time. It looked to us like the transmission was within the facility. There was a case, and then another a few days later."
Health investigators have tested the 22 children, as well as four adults, Melnick said. Seven additional people, a mix of staff and children, showed no symptoms but had E. coli in their stool, he added.
Symptoms can taken as long as 10 days to appear after exposure so the health department is checking with staff and the children's families daily.
Melnick said the day care will remain closed until affected staff show no presence of the bacteria on two consecutive tests. Children who tested positive will have to meet the same criteria before being allowed to attend any day care or school, he said.