U.S. Department of Homeland Security Secretary Kirstjen Nielsen has resigned after 16 months on the job and immense pressure from the White House and public over the situation along the country’s southern border.

“Despite our progress in reforming homeland security for a new age, I have determined that it is the right time for me to step aside,” Nielsen wrote in her resignation letter to President Donald Trump.

While she did not say exactly why she is quitting, Nielsen wrote she hopes 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. Our country and the men and women of DHS deserve to have all the tools and resources they need to execute the mission entrusted to them.”

Nielsen said later on Twitter her last day on the job will be Wednesday.

Trump has expressed frustration with the situation along the U.S. border with Mexico, where hundreds of thousands of migrants trying to escape poverty and crime in central America have traveled in hopes of entering the United States.

He reiterated his position in a series of Twitter comments late Sunday, punctuating his comments with “Our Country is FULL!”

Trump again threatened to impose tariffs on Mexican goods or close the border if, in his words, Mexican authorities do not “apprehend all illegals.” Under U.S. law, foreign nationals are allowed to apply for asylum.

White House sources have said Trump often yelled at Nielsen for apparently not being strong enough in curbing the number of migrants trying to enter the United States.

House Speaker Nancy Pelosi called the Trump administration’s immigration polices “cruel” and said they “have only worsened the humanitarian suffering at the border.” 

“It is deeply alarming that the Trump Administration official who put children in cages is reportedly resigning because she is not extreme enough for the White House’s liking,” she said. “America needs a Homeland Security Secretary who will respect the sanctity of families, honor our proud heritage as a nation of immigrants, and restore sanity to this Administration’s policies.”

Nielsen faced a public outcry over the administration’s highly unpopular policy of separating migrant families when they crossed into the U.S.

Thousands of young children were taken from their parents and held in separation facilities. Nielsen was responsible for executing that policy while at times denying there was such a policy.

Despite the acrimony, Nielsen wrote, “I can say with confidence our homeland is safer today than when I joined the administration. We have taken unprecedented action to protect Americans.”

Trump thanked Nielsen for her service in his own Twitter post, while also announcing that current U.S. Customs and Border Protection Commissioner Kevin McAleenan will become acting DHS secretary. 

“I have confidence that Kevin will do a great job,” Trump said.