Acting U.S. Customs and Border Protection chief John Sanders told employees on Tuesday that he is resigning amid increased pressure to improve conditions at migrant detention centers along the U.S.-Mexico border.

In a statement about his resignation, Sanders said, “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 his CBP has been the most fulfilling and satisfying opportunity of my career.”

The recent influx of immigrants attempting to cross the border from Mexico in the United States has put immense pressure on border officials to properly house and feed the hundreds of refugees who are being held in American detention centers. Recent reports have shown that immigrants are being forced to live in squalid conditions with inadequate food and contaminated water. The shelters are so overcrowded that not only are there an insufficient number of beds, but there isn’t even enough space for all detainees to sleep inside, forcing some migrants to lie outside in the cold.

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Particular attention has been drawn to the plight of migrant children, who are often forcibly separated from their parents and family members. Rep. Veronica Escobar (R-Texas) reportedly sent a letter to Sanders bashing him for the harsh treatment that many of the detained children were receiving. “Children do not belong in detention and @realDonaldTrump’s failed policies are only harming children and promoting needless and cruel family separation,” she wrote in a tweet.

This situation has proved near impossible for border officials to navigate, with various leaders from organizations such as Immigration and Customs Enforcement and CBP resigning in the face of such pressure. Acting Immigration and Customs Enforcement Director Mark Morgan will be replacing Sanders as the head of CBP. During his brief tenure at ICE, Morgan was responsible for planning the mass crackdown on illegal immigrants in the United States that was to begin last week before President Donald Trump called it off.