Read: Kirstjen Nielsen's farewell message to DHS employees

(CNN)President Donald Trump announced on Twitter Sunday that Kirstjen Nielsen, the secretary of Homeland Security, is leaving his administration.

In a farewell message to DHS employees, Nielsen says she is confident in Acting Secretary Kevin McAleenan, who was appointed as her temporary replacement.
Details of Nielsen's letter to employees are below:
Farewell Message from Secretary Kirstjen M. Nielsen
    As you know, I submitted my resignation to the President earlier this week. Today is my final day at the Department. For nearly two years, I've had the honor to work alongside a talented, dedicated workforce. You serve with professionalism, respect, and determination in fulfilling DHS' important missions. The commitment you make day in and out -- securing the land air and sea domains, preventing terrorism, administrating immigration laws, ensuring disaster resilience, safeguarding our cybersecurity landscape and infrastructure, and so much more -- is awe inspiring.
    Your work is vital to making every American safer, and as I leave you, know that I'll always admire what you've done to defend our way of life. Customs and Border Protection (CBP) Commissioner Kevin McAleenan will now lead DHS as your Acting Secretary. Kevin has led the Department's largest component for two years and his qualifications and experience will be instrumental in overseeing the crisis at our nation's borders. I have full confidence in Kevin's ability to carry out the DHS mission of safeguarding the American people, our homeland, and our values. Please join me in welcoming Kevin as the Acting Secretary.
    To each of you I am eternally grateful for your service and could not be prouder to have served alongside you. Thank you for your devotion and sacrifice. God bless each of you and your families, and God bless this great country.
    Warmest regards,
    Kirstjen M. Nielsen
    Secretary of Homeland Security
    In his first message to DHS employees as acting Secretary, Kevin McAleenan thanks Nielsen.
    In the message, obtained by CNN's Jessica Schneider, he also mentions the border issues, saying, "While we face many challenges, including a growing crisis on our southern border, I know that, working together, we can overcome them."
    Details of McAleenan's letter to employees are below:
    Message from Acting Secretary Kevin K. McAleenan
    As I assume the role of Acting Secretary of Homeland Security, I look forward to working to support the men and women of the Department of Homeland Security (DHS) as they carry out the critical missions that keep our country safe. I'd also like to extend my sincere thanks to Secretary Kirstjen Nielsen for her service to the country as the sixth Secretary of Homeland Security, and for her tireless work on behalf of the people and organizations of DHS. She deserves our enduring respect and appreciation for her leadership, and I'm personally grateful for it.
    In many roles in my career at U.S. Customs and Border Protection, I've had the opportunity to work with virtually every component of DHS, and have witnessed the extraordinary commitment and steadfast resolve of the men and women across this Department. While we face many challenges, including a growing crisis on our southern border, I know that, working together, we can overcome them. I've also learned that the true strength of DHS is our people, and during my time as Acting Secretary, I pledge to be an aggressive advocate for you.
    I look forward to working alongside you to accomplish the vast mission of securing our homeland with honor, integrity and vigilance. Let's power on. The American people are counting on us.
    Kevin K. McAleenan
      Acting Secretary of Homeland Security