WASHINGTON, DC - MAY 15: Customs and Border Protection acting Commissioner John Sanders speaks at a news conference proposing legislation to address the crisis at the southern border at the U.S. Capitol on May 15, 2019 in Washington, DC. Senate Republicans met with Vice President Mike Pence and White House Advisor Jared Kushner yesterday to discuss President Donald Trump's plan to overhaul the immigration system. (Photo by Anna Moneymaker/Getty Images)
Anna Moneymaker/Getty Images
WASHINGTON, DC - MAY 15: Customs and Border Protection acting Commissioner John Sanders speaks at a news conference proposing legislation to address the crisis at the southern border at the U.S. Capitol on May 15, 2019 in Washington, DC. Senate Republicans met with Vice President Mike Pence and White House Advisor Jared Kushner yesterday to discuss President Donald Trump's plan to overhaul the immigration system. (Photo by Anna Moneymaker/Getty Images)
Now playing
01:41
Acting US Customs and Border Protection chief to resign
ST. PAUL, MN - NOVEMBER 6:  Former U.S. Vice President Walter Mondale concedes the election to his Republican opponent Norm Coleman November 6, 2002 in St. Paul, Minnesota. Mondale and Coleman were in a race for U.S. Senate that was too close to call the evening before.  (Photo by Mark Erickson/Getty Images)
Mark Erickson/Getty Images
ST. PAUL, MN - NOVEMBER 6: Former U.S. Vice President Walter Mondale concedes the election to his Republican opponent Norm Coleman November 6, 2002 in St. Paul, Minnesota. Mondale and Coleman were in a race for U.S. Senate that was too close to call the evening before. (Photo by Mark Erickson/Getty Images)
Now playing
03:00
Walter Mondale dies at 93
george w bush congress immigration rhetoric cbs intv sot mxp vpx_00000000.png
george w bush congress immigration rhetoric cbs intv sot mxp vpx_00000000.png
Now playing
01:25
Bush calls on Congress to tone down 'harsh rhetoric' on immigration
WASHINGTON, DC - MARCH 23: Sen. Ted Cruz (R-TX) speaks at a Senate Judiciary Committee hearing on "Constitutional and Common Sense Steps to Reduce Gun Violence" on March 23, 2021 in Washington, DC.  Many senators spoke both for and against gun control the day after a shooting in Boulder, Colorado where a gunman opened fire at a grocery store, killing ten people. (Photo by Tasos Katopodis/Getty Images)
Tasos Katopodis/Getty Images
WASHINGTON, DC - MARCH 23: Sen. Ted Cruz (R-TX) speaks at a Senate Judiciary Committee hearing on "Constitutional and Common Sense Steps to Reduce Gun Violence" on March 23, 2021 in Washington, DC. Many senators spoke both for and against gun control the day after a shooting in Boulder, Colorado where a gunman opened fire at a grocery store, killing ten people. (Photo by Tasos Katopodis/Getty Images)
Now playing
03:18
Berman on Cruz's latest tweet: 'The pot calling the kettle violent'
Now playing
01:57
Chuck Hagel criticizes Trump's statement on Afghanistan
gun laws shootings Comer pamela brown nr vpx _00015627.png
CNN
gun laws shootings Comer pamela brown nr vpx _00015627.png
Now playing
02:23
'I can't answer that': Kentucky lawmaker responds to CNN on gun policy
Now playing
02:39
National security adviser: Russia will face consequences if Navalny dies in prison
Rep. Marjorie Taylor Greene (R-GA) speaks during a press conference outside the U.S. Capitol on February 5, 2021 in Washington, DC. The House voted 230 to 199 on Friday evening to remove Rep. Marjorie Taylor Greene (R-GA) from committee assignments over her remarks about QAnon and other conspiracy theories.
Drew Angerer/Getty Images
Rep. Marjorie Taylor Greene (R-GA) speaks during a press conference outside the U.S. Capitol on February 5, 2021 in Washington, DC. The House voted 230 to 199 on Friday evening to remove Rep. Marjorie Taylor Greene (R-GA) from committee assignments over her remarks about QAnon and other conspiracy theories.
Now playing
03:20
Marjorie Taylor Greene lashes out at media after backlash over controversial caucus
AP
Now playing
03:16
Maxine Waters: Jim Jordan is a bully and I shut him down
US Representative Marjorie Taylor Greene, Republican of Georgia, leaves her office on Capitol Hill on February 4, 2021 in Washington, DC.
NICHOLAS KAMM/AFP/Getty Images
US Representative Marjorie Taylor Greene, Republican of Georgia, leaves her office on Capitol Hill on February 4, 2021 in Washington, DC.
Now playing
03:51
Marjorie Taylor Greene launching 'America First' caucus
US President Joe Biden delivers remarks on Russia at the White House in Washington, DC on April 15, 2021. - The United States announced sanctions and the expulsion of 10 Russian diplomats Thursday in retaliation for what Washington says is the Kremlin's US election interference, a massive cyberattack and other hostile activity.
JIM WATSON/AFP/Getty Images
US President Joe Biden delivers remarks on Russia at the White House in Washington, DC on April 15, 2021. - The United States announced sanctions and the expulsion of 10 Russian diplomats Thursday in retaliation for what Washington says is the Kremlin's US election interference, a massive cyberattack and other hostile activity.
Now playing
02:22
White House backtracks on refugees decision after criticism
Biden speaks from the Treaty Room in the White House on April 14, 2021 in Washington, DC.
Andrew Harnik/Pool/Getty Images
Biden speaks from the Treaty Room in the White House on April 14, 2021 in Washington, DC.
Now playing
02:44
'National embarrassment': Biden reacts to mass shootings
MOSCOW, RUSSIA - APRIL 15:  Russian President Vladimir Putin (L) talks to talks to Foreign Minister Sergey Lavrov during a meeting with Uzbekistan President Islam Karimov in the Kremlin on April 15, 2013 in in Moscow, Russia. Karimov is on a state visit to Russia. (Photo by Sasha Mordovets/Getty Images)
Sasha Mordovets/Getty Images
MOSCOW, RUSSIA - APRIL 15: Russian President Vladimir Putin (L) talks to talks to Foreign Minister Sergey Lavrov during a meeting with Uzbekistan President Islam Karimov in the Kremlin on April 15, 2013 in in Moscow, Russia. Karimov is on a state visit to Russia. (Photo by Sasha Mordovets/Getty Images)
Now playing
02:07
Russia to expel 10 US diplomats in 'tit-for-tat response' to Biden sanctions
Now playing
03:10
Avlon: Here's what we know 100 days since the Capitol riot
A Russian flag flies next to the US embassy building in Moscow on October 22, 2018. - US national security advisor John Bolton is in Moscow holding meetings with senior Russian officials following Washington's weekend announcement of withdrawal from the Cold War-era Intermediate-Range Nuclear Forces Treaty, known as the INF. (Photo by Mladen ANTONOV / AFP)        (Photo credit should read MLADEN ANTONOV/AFP via Getty Images)
Mladen Antonov/AFP/Getty Images
A Russian flag flies next to the US embassy building in Moscow on October 22, 2018. - US national security advisor John Bolton is in Moscow holding meetings with senior Russian officials following Washington's weekend announcement of withdrawal from the Cold War-era Intermediate-Range Nuclear Forces Treaty, known as the INF. (Photo by Mladen ANTONOV / AFP) (Photo credit should read MLADEN ANTONOV/AFP via Getty Images)
Now playing
02:17
Political scientist: US-Russia relations are in the toilet
WASHINGTON, DC - JUNE 13: Rep. Michelle Lujan Grisham (D-NM) speaks during a news conference on immigration to condemn the Trump Administration's "zero tolerance" immigration policy, outside the US Capitol on June 13, 2018 in Washington, DC. (Photo by Toya Sarno Jordan/Getty Images)
Toya Sarno Jordan/Getty Images
WASHINGTON, DC - JUNE 13: Rep. Michelle Lujan Grisham (D-NM) speaks during a news conference on immigration to condemn the Trump Administration's "zero tolerance" immigration policy, outside the US Capitol on June 13, 2018 in Washington, DC. (Photo by Toya Sarno Jordan/Getty Images)
Now playing
02:39
Governor settles with former campaign staffer who accused her of sexual mistreatment
(CNN) —  

Acting Customs and Border Protection Commissioner John Sanders is resigning, he said in a message sent to agency employees Tuesday, amid the dramatic increase in the number of undocumented migrants crossing the border, a fight over how to address it and controversy over how children are being treated.

“Although I will leave it to you to determine whether I was successful, I can unequivocally say that helping support the amazing men and women of CBP has been the most fulfilling and satisfying opportunity of my career,” Sanders writes. His resignation is effective July 5.

Acting Immigration and Customs Enforcement Director Mark Morgan is expected to take over as Customs and Border Protection in an acting capacity, according to a Department of Homeland Security official. Sanders’s resignation as acting head of CBP comes amid a crush of migrants at the border that has overwhelmed facilities. Earlier Tuesday, CBP held a call with reporters on squalid conditions at a Border Patrol facility in Clint, Texas.

Officials conceded that children should not be held in CBP custody, noting that the agency’s facilities were designed decades ago to largely accommodate single adults for a short period of time.

The Washington Post first reported Morgan’s move.

Over the weekend, President Donald Trump called off planned raids by US Immigration and Customs Enforcement, saying deportations would proceed unless Congress finds a solution on the US-Mexico border within two weeks. Before it was postponed, Mark Morgan had publicly confirmed an operation targeting migrant families and others with court-ordered removals was in the works.

Morgan, a vocal proponent of the President’s efforts, was another of Trump’s picks to lead ICE after abruptly pulling the nomination of Ron Vitiello.

Morgan briefly served as Border Patrol chief during the Obama administration before leaving the post in January 2017. He previously spent two decades at the FBI. He is expected to return to Customs and Border Protection, which encompasses Border Patrol.

Sanders assumed the post after Kevin McAleenan, the former commissioner, moved up to fill the role of acting homeland security secretary in the wake of Kirstjen Nielsen’s ouster this spring. In his role, Sanders has overseen the agency responsible for policing the US borders and facilitating legal trade and travel. It is also the frontline agency dealing with the surge of migrants at the southern border.

Robert Perez, the highest-ranking career official, is the current deputy commissioner. It is unclear if he will step into the acting commissioner position.

Before becoming acting commissioner, Sanders, served as the Chief Operating Officer at CBP, where he worked with McAleenan to address the operational needs of the agency and work on strategic direction.

As of June 1 this fiscal year, Border Patrol has arrested more than 377,000 family units, over 60,000 unaccompanied children, and over 226,000 single adults.

Sanders did not provide a reason for his departure.

Read Sanders’s letter here:

As some of you are aware, yesterday I offered my resignation to Secretary McAleenan, effective Friday, July 5. In that letter, I quoted a wise man who said to me, “each man will judge their success by their own metrics.” Although I will leave it to you to determine whether I was successful, I can unequivocally say that helping support the amazing men and women of CBP has been the most fulfilling and satisfying opportunity of my career.

I’ve spent a significant amount of time over the last several days reflecting on my time at CBP. When I began this journey, Commissioner McAleenan charged me with aligning the mission support organizations and accelerating his priorities. Easy enough, I thought. What I didn’t appreciate at the time was how the journey would transform me professionally and personally. This transformation was due in large part to the fact that people embraced and welcomed me in a way that was new to me – in a way that was truly special. To this day, I get choked up when speaking about it and I can’t adequately express my thanks. As a result, let me simply say I will never stop defending the people and the mission for which 427 people gave their lives in the line of duty in defending. Hold your heads high with the honor and distinction that you so richly deserve.

Throughout our journey together, your determination and can-do attitude made the real difference. It allowed CBP to accomplish what others thought wasn’t possible…what others weren’t able to do. And even though there is uncertainty during change, there is also opportunity. I therefore encourage everyone to reflect on all that you have accomplished as a team. My hope is you build upon your accomplishments and embrace new opportunities, remain flexible, and continue to make CBP extraordinary. This is your organization…own it! Don’t underestimate the power of momentum as you continue to tackle some of this country’s most difficult challenges.

I will forever be honored to have served beside you. As a citizen of this great country, I thank you for your public service.

Take care of each other,

John

This story has been updated.