The Labor Department on Thursday decried a national surge in child labor, saying that the agency’s inspectors had found thousands of violations and were investigating a slaughterhouse where a 16-year-old boy from Guatemala was killed this month.

The update followed a hearing on Wednesday in which lawmakers from both parties accused the Health and Human Services secretary, Xavier Becerra, of failing to protect migrant children from exploitation. His agency is tasked with releasing them to safe living conditions after they cross the border by themselves.

“There are some terrible things that are wrong,” Representative Anna G. Eshoo, a Democrat from California, told him. “At the end of the day, as H.H.S. secretary, the buck stops with you.”

Some 300,000 minors have come to this country alone since 2021, fueling a dramatic increase in migrant child labor. In an online report, the Labor Department announced an 87 percent increase in fines on employers in recent months. Companies including lumber mills and roofing contractors have been hit with $6.6 million in penalties.

The department said that inspectors were pursuing more than 700 open cases and had already found 4,474 children working illegally since the start of the fiscal year — a 44 percent increase over the previous year.

The agency vowed to ramp up enforcement and create a child labor task force in February, days after The New York Times reported that unaccompanied minors were being forced into some of the nation’s most grueling jobs.

The Times found cases of 12-year-olds working construction, 13-year-olds washing hotel sheets and 15-year-olds packaging Cheerios overnight. One boy was released to a man in Florida who promised to put him in school but instead threatened him, demanding money. “Don’t mess with me,” the man wrote. “You don’t mean anything to me.”

These children were released to sponsors by the H.H.S. shelter system, which is supposed to pair children with responsible adults. As more and more minors began entering the country after President Biden took office, his administration ramped up pressure on the agency to place the children more quickly. It rolled back protections that had been in place for years, including some background checks, and Mr. Becerra urged staff members to move with the speed of a Henry Ford assembly line.

On Wednesday, the House Committee on Energy and Commerce questioned Mr. Becerra for three hours about the rising number of children falling prey to exploitation.

Lawmakers also pressed him on instances of retaliation against whistle-blowers. Workers said they had tried to warn Mr. Becerra and the White House that migrant children were being sent to unsafe homes, only to be punished for speaking out. One worker said her building access was revoked during her lunch break after she raised alarms about a 14-year-old whose sponsor made him work off a debt.

“Thanks to The New York Times, we know that reports reached your desk that children were at risk,” said the committee chair, Representative Cathy McMorris Rodgers, a Republican from Washington. “The American people must know how you could possibly have ignored these warning signs.”

Mr. Becerra told lawmakers that his department was increasing its services for children. “This is real, it’s repulsive and it’s unacceptable,” he said.

A new accountability team at H.H.S. will try to spot problems with the vetting of sponsors. For years, the agency released most children with little follow-up beyond a phone call. But now, the department says, it is on track to give all children case management services by 2024.

Lawmakers from both parties have introduced bills to increase civil penalties for child labor violations. But these efforts have stalled amid partisan gridlock, and politicians have begun looking for other ways to apply pressure. House Democrats last week launched a task force to fight child labor and also requested a federal audit to quantify the scope of the problem.

The Labor Department said it would continue to focus on finding companies that rely on child workers and also to hold brands accountable, not just their manufacturers and staffing agencies. The agency has stepped up its use of the “hot goods” legal provision, which can prohibit any interstate transport of products when child labor is involved. The department used the rule last month on a Minnesota factory run by Monogram Meat Snacks, a national supplier of jerky.

In a statement, Monogram said it had “immediately terminated the two ineligible workers” and had made “companywide changes” to prevent underage labor in the future.

Last year, a 10th grader from Central America told The Times that many young teenagers worked at Monogram when they first arrived in the United States until they could find better jobs. He had worked there himself, he said, until he found a higher-paying job sawing planks of wood each night until 5 a.m. He needed the extra money to catch up on the rent he owed his sponsor.