US Homeland Security Secretary Kirstjen Nielsen, who oversaw President Donald Trump’s bitterly contested immigration policies during her tumultuous 16-month tenure, resigned on Sunday amid a surge in the number of migrants at the border with Mexico.

Her departure had been repeatedly rumored over the past year, particularly after a wave of anger over the administration’s 2018 family separation policy at the border with Mexico and most recently as US border officials estimated that 100,000 migrants were apprehended at the southern border in March, the highest level in a decade.

Another senior administration official said Trump’s national security adviser, John Bolton, after a blowup with Nielsen late last year, also recommended to Trump that she should go.Trump has made a clampdown on illegal immigration a centerpiece of his two-year-old presidency, leading chants of “Build that wall” at his rallies as he has sought to cut back on the number of newcomers entering the United States without proper documentation.

Trump was so frustrated about the increase that he announced he would cut off US aid to Guatemala, Honduras and El Salvador. He also threatened to close the border with Mexico, although he later backed off that proposal with a threat to impose tariffs on auto imports.

In her resignation letter, Nielsen asked for more from Congress and the courts, which have opposed such Trump administration initiatives as his effort to limit immigration from Muslim nations and the border wall.

“I hope that the next Secretary will have the support of Congress and the courts in fixing the laws which have impeded our ability to fully secure America’s borders and which have contributed to discord in our nation’s discourse,” she wrote to Trump.