“We had one. Then we had another one, then we had another one,” Mr. Hughes said. Patients were falling ill, and deteriorating with troubling speed. “The nurses kept saying, ‘They were not like this two hours ago.’”

Neither the paramedic crews nor the sick residents had been wearing masks or other protection. As they loaded up patient after patient, Lieutenant Hughes thought to himself: This is way too many.

The nursing home started to discourage visitors, but it did not forbid them, and family members said they did not think anything was seriously wrong.

“I didn’t see anything,” said Amy Jou, who visited on Feb. 28 to do her 93-year-old mother’s laundry.

Ms. Neidigh came that same morning to bring her mother coffee and discuss plans for moving her back into her own apartment. She said the staff warned her about what it still thought was a respiratory outbreak, and urged her to wear a mask. She slipped one on, but since several staff members were not wearing protection, she said, she figured there was little to worry about.

Mr. Killian, the spokesman, said that while some administrators or reception staff may not have been wearing protection at that point, the workers in contact with patients were all in masks. “Of course we were geared up,” he said. “Of course we were.”

The first coronavirus case would be confirmed later that night.