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It was snowing, the heater in my rental car had broken, and I felt somewhat shady as I studied the faces of workers leaving a factory at midnight in Grand Rapids, Mich., last fall.

I had come to the heart of the industrial Midwest because I thought I might find migrant children working dangerous jobs. As the late shift ended and people in hairnets began to emerge from the building, I saw that many of them were short and baby faced. But there was another problem: The workers didn’t have coats and they were running through a November cold snap to find their car pools. I asked myself how I was going to talk to them without seeming like a stranger chasing children through the snow.

This was my last stop for a project that had led me to seven states and entailed interviews with more than 100 migrant children working exploitative jobs across the country.

In the last two years, more than 250,000 children have entered the United States by themselves, many of them from Central America. I found that thousands of them had ended up in punishing jobs that flouted child labor laws. Children as young as 12 and 13 year olds are working in commercial laundromats, industrial bakeries and sock mills.

I started my reporting back in the spring of 2022, shortly after I joined The Times. I mentioned to my editor, Kirsten Danis, that I had once come across a group of migrant children working in a cookie factory on Long Island. I never followed up on what I had seen, but Kirsten’s surprise helped me see that there might be a larger story there.

My first step was to request a database of information about migrant children through the Freedom of Information Act. After the government was slow to respond, The Times sued for these and other records. The data I eventually obtained revealed a trove of details about children who had come to this country since 2021, including where they were living and whether they had been reunited with their parents.

When minors cross the border alone, the government is supposed to release them to trustworthy adults. A decade ago, most children went to a parent already living in this country. But now a majority go to more distant relatives, acquaintances or even strangers because their parents are not in the United States. I saw that in northwest Grand Rapids, only 7 percent of migrant children were being released to parents. It was an immediate red flag.

One of my first stops when I got to the city was Union High School, which I learned had a thriving program for English-language learners. In a ninth-grade class, I saw three girls whispering with each other as their teacher played a movie. They told me they all worked overnight packing cereal into boxes. One of them added, “My uncle says that he’s going to let me stop soon.”

I felt terrible for these children. But I believed them, because I had spoken to so many young people leading similar lives at that point. I asked them about their schedules and the company they worked for. They said they always car-pooled to work with an adult, and had not paid attention to the specifics about their employer.

So I turned to a community organizer, who told me there was only one cereal factory in the city known for hiring children. It was called Hearthside Food Solutions, and it made and packaged food for larger companies. She connected me with a Hearthside worker who knew what brand the company was running on its assembly lines that night: Cheerios.

It is a clear violation of child labor laws for young teenagers to work with industrial machines, and I wondered if General Mills, which owns Cheerios, would really let such an iconic brand get mixed up with child labor. (In reviewing The Times’s findings from my investigation, General Mills has said it recognizes “the seriousness of this situation.” Hearthside said in a statement that it was “deeply concerned” and had prohibited its suppliers from hiring underage workers.) I needed to check the situation out. I killed time until the late shift ended, then drove to the factory and tried to avoid parking under the floodlights.

I had not expected to see any children exit the factory, but after a few minutes I saw a group of workers who not only looked underage, they looked exactly like the girls I had spoken to in class that afternoon. They waved to me in recognition as they walked by.

I made two more trips to Grand Rapids and eventually met and exchanged phone numbers with many more young workers. Later, I followed up with their adult sponsors in the United States and parents in Central America to make sure we had additional consent to have them featured in the article. I reviewed the children’s legal documents to verify their ages, and looked at their company badges and paychecks to verify their jobs.

Like most teenagers, they were constantly documenting their lives, and several had taken photos and videos at the plants. I learned that inside, the factories were harshly lit and sounded like an airplane taking off.

On Sundays — often their only day off — they sacrificed catching up on sleep to talk with me. Children in this shadow work force rarely speak out. They tend to be wary of outsiders who ask questions about their ages and jobs. But families decided to go on the record because they felt it was important to show what life was really like for migrant children in the United States. Many children told me they wished they had known this themselves before heading north.

They had not anticipated policy changes in response to these stories, but two days after this investigation was published, the White House announced a series of immediate reforms at two agencies and launched a federal task force.

I checked in this week with one teenager who had been working in Grand Rapids making auto parts used by Ford and General Motors. (The companies said they took child labor violations at suppliers seriously and were investigating.) I caught him while he was driving. His company had fired all of the minors, he said. But there was still work at another plant nearby. Then he told me he had to go — he had just arrived and his shift would be starting soon.