Sen. Barbara Boxer (D-CA) called on federal authorities Wednesday to investigate allegations that Donald Trump’s model agency knowingly employed models who were working in the United States without proper visas.

In a letter to U.S. Citizenship and Immigration Services Director León Rodríguez and Labor Secretary Tom Perez, Boxer wrote that she was “extremely concerned” about reports indicating “widespread noncompliance with immigration and labor laws by Trump Model Management.”

“I hope you will make clear that immigration and labor violations like these will not be tolerated,” Boxer wrote.

In August, Mother Jones reported that former models said Trump’s agency knew that they were working in the country without proper documentation.

Trump campaign spokeswoman Hope Hicks declined to answer detailed questions about the story, according to Mother Jones, and Trump Model Management has not commented on the allegations in the initial report.

Boxer, a vocal critic of the GOP candidate and a close ally of Hillary Clinton, took to Twitter last week to blast Trump over the report as he headed to Mexico:

Apparently Trump never built a wall to keep foreign models from working illegally in this country for low pay. https://t.co/xAMzTWba0c — Barbara Boxer (@BarbaraBoxer) August 31, 2016

Read the full letter below via Mother Jones: