A child who worked for Packers Sanitation Services cleans a slaughterhouse in Grand Island, Neb. The subject has been blurred by the source.

“[E]xamine your own child labor practices and ensure you are compliant with federal labor laws,” wrote Sens. Padilla and Hickenlooper to Target, J.Crew, General Mills and 24 others.

In the wake of federal investigations into illegal child labor and media reports including an NBC News series on children cleaning slaughterhouses, two Democratic senators are sending letters to 27 CEOs demanding responses on their companies’ compliance with labor laws.

The new Senate inquiry is taking place amid a 69% increase in the number of children found to be employed in violation of labor laws, according to the Department of Labor.

In the letters, Democrats Alex Padilla of California and John Hickenlooper of Colorado wrote: “We write to express our deep concern regarding recent public reporting of labor exploitation of child migrants. Many unaccompanied children reported to be working in unsafe conditions have already endured horrific trauma in making the perilous journey to the United States, and we ask that you closely examine your own child labor practices and ensure you are compliant with federal labor laws to avoid putting more children in harm’s way.”

The letters note that according to federal labor laws, children can only work three hours a day on school days and 18 hours total each week from 7 a.m. to 7 p.m.

In mid-February, the Labor Department found more than 100 children illegally working dangerous jobs on overnight shifts for Packers Sanitation Services Inc. (PSSI) at 13 slaughterhouses in eight states, including a JBS Foods plant. Some children were as young as 13 and were attending middle school during the day. The company entered into an agreement with the agency pledging compliance with child labor laws and consented to third-party oversight. The company also paid a $1.5 million fine.

The companies receiving letters Wednesday morning include General Mills, Packers Sanitation Services Inc., JBS Foods, Hearthside Foods, J.Crew and Target.

This is the second Senate inquiry citing NBC News reporting on child labor. Last week, a bipartisan inquiry was sent to PSSI by Sen. Bernie Sanders, I-Vt., chairman of the Health, Education, Labor and Pensions Committee, and ranking Sen. Bill Cassidy, R-La., asking the company to show how it had changed business practices since the consent decree.

In response to the Labor Department’s investigation, PSSI told NBC News in a statement: “We have been crystal clear from the start: Our company has a strong corporate commitment to our zero-tolerance policy against employing anyone under the age of 18, and fully shares the DOL’s objective of ensuring full compliance at all locations. As parents and citizens, we don’t want a single person under 18 working for PSSI, period — and take extensive steps to prevent individuals at the local level from circumventing our wide-ranging procedures.”

PSSI is owned by the private equity firm Blackstone. In early March, New York State Comptroller Thomas DiNapoli sent a letter to the firm asking how it planned to “implement strict measures that would prevent this from occurring in the future.”

DiNapoli’s staff said they received two letters from Blackstone CEO Steve Schwarzman in response and had some conversations with their staff. DiNapoli’s spokesperson declined to share Schwarzman’s response but described the conversations as “frank and productive.”

Blackstone provided NBC News with the text of its response, which said in part, “Blackstone and PSSI have been crystal clear from the start: PSSI has a strong corporate commitment to its zero-tolerance policy against employing anyone under the age of 18, and we all fully share the DOL’s objective of ensuring full compliance at all locations. As parents and citizens, we don’t want a single person under 18 working for PSSI, period — and PSSI takes wide-ranging steps to help prevent individuals at the local level from circumventing their processes in this area.”

The Senate letters to CEOs also cite the New York Times coverage of child labor, including alleged child labor at suppliers to Target and J.Crew and the paper’s exposé on children who were hired to work at Hearthside Food Solutions in Michigan.

J.Crew told the Times that it “took the allegations seriously and would investigate.” Target did not directly respond to the Times but the company’s website cites a “zero-tolerance policy for underage labor.”

General Mills issued a statement saying, “General Mills has no tolerance for any of our suppliers illegally employing or harassing minors. We are actively reviewing this situation, which allegedly occurred outside of a General Mills owned and operated plant… We recognize the seriousness of this situation and will take appropriate action based on our findings.”

JBS Foods has told other outlets it is taking the issue seriously. A spokesperson told CBS News, “We are immediately launching an independent, third-party audit at all of our facilities to thoroughly evaluate this situation. JBS has zero tolerance for child labor, discrimination or unsafe working conditions for anyone working in our facilities. We expect and contractually require our partners to adhere to the highest ethical principles as outlined in our business associate code of conduct.”

A former employee who worked at the Hearthside factory and left her employment six months ago told NBC News in a recent interview that she was troubled by the children she regularly saw working in the Grand Rapids factory where the company makes chocolate Cheerios products for General Mills. She requested that NBC News not identify her.