In the spring of 2021, Linda Brandmiller was working at an arena in San Antonio that had been converted into an emergency shelter for migrant children. Thousands of boys were sleeping on cots as the Biden administration grappled with a record number of minors crossing into the United States without their parents.

Ms. Brandmiller’s job was to help vet sponsors, and she had been trained to look for possible trafficking. In her first week, two cases jumped out: One man told her he was sponsoring three boys to employ them at his construction company. Another, who lived in Florida, was trying to sponsor two children who would have to work off the cost of bringing them north.

She immediately contacted supervisors working with the Department of Health and Human Services, the federal agency responsible for these children. “This is urgent,” she wrote in an email reviewed by The New York Times.

But within days, she noticed that one of the children was set to be released to the man in Florida. She wrote another email, this time asking for a supervisor’s “immediate attention” and adding that the government had already sent a 14-year-old boy to the same sponsor.

Ms. Brandmiller also emailed the shelter’s manager. A few days later, her building access was revoked during her lunch break. She said she was never told why she had been fired.

Over the past two years, more than 250,000 migrant children have come alone to the United States. Thousands of children have ended up in punishing jobs across the country — working overnight in slaughterhouses, replacing roofs, operating machinery in factories — all in violation of child labor laws, a recent Times investigation showed. After the article’s publication in February, the White House announced policy changes and a crackdown on companies that hire children.

But all along, there were signs of the explosive growth of this labor force and warnings that the Biden administration ignored or missed, The Times has found.