“How do you take a kid in for having respiratory problems and you don’t test him?” he said, using a vulgarity. “I am so livid.”

The teenager, whose name has not been released, was then treated at a second hospital, where he died, the mayor said. The positive test results did not come back until after his death, and it was unclear when the test was taken.

The boy’s father, an Uber driver, is also sick, Mr. Parris said.

“I’m really upset right now. I thought this was a disease that affected people my age,” said Mr. Parris, 68. “Now I realize our kids are at risk — that’s something totally different.”

He said that children in Lancaster are now ordered to stay at home, and that if they were found outside, they would be taken back to their homes. “We are enforcing the law in Lancaster,” he said. “I will not lose another child.”

On Tuesday, the Los Angeles County Department of Public Health reported three deaths linked to the coronavirus, including the teenager, bringing the total number of deaths in the area to 11. Officials said that two people between the ages of 50 and 70, one with underlying heath conditions, had also died.

“Each loss we experience in L.A. County is tragic, and we are sending our heartfelt condolences to the families and loved ones who’ve had to endure this tragedy,” Barbara Ferrer, director of the Los Angeles County Department of Public Health, said in a statement. “Covid-19 does not discriminate by age, race or income level.”

Officials said 256 new cases had been reported across Los Angeles County over the last two days, bringing the total number of cases there to 662. In the United States, as of Tuesday night, there have been at least 53,478 cases of coronavirus confirmed by lab tests and 714 deaths, according to a New York Times database.