President Trump announced Sunday afternoon that Homeland Security Secretary Kirstjen Nielsen “will be leaving her position” after 16 months in the job.

Nielsen tweeted Sunday evening that she had submitted her resignation and added: “Its [sic] been an honor of a lifetime to serve with the brave men and women of @DHSgov. I could not be prouder of and more humbled by their service, dedication, and commitment to keep our country safe from all threats and hazards.” She included an image of a resignation letter to Trump in which she wrote: “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.”

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“Our missions as a Department are vast and have never been more vital,” Nielsen wrote. “You are in the arena- keep up the good fight. Thank you for your sacrifices and those of your families. God bless you and God bless our great country.”

Nielsen met with Trump at the White House Sunday amid an ongoing influx of migrants at the U.S.-Mexico border that has been taxing America’s immigration system and sparking frustration within the administration. The Associated Press, citing two sources, reported that Nielsen had been frustrated with the difficulty of getting other departments to help deal with the growing number of families crossing the southwestern border.

Administration sources told Fox News Sunday evening that Nielsen’s background in cybersecurity made her a poor fit to handle border issues, while McAleenan best fits Trump’s requirement of being the “toughest cop” on the frontier.

(Excerpted from Fox News, Samuel Chamberlin reporting.)