Outgoing White House chief of staff John Kelly says former Attorney General Jeff Sessions is to blame for the Trump administration’s controversial “zero-tolerance” immigration policy, which separated children from parents who illegally crossed into the U.S. from Mexico.

Kelly spoke out about the policy ― as well as his tumultuous tenure in the White House ― in an interview published in the Los Angeles Times on Sunday.

The retired Marine general told the Times that the White House had been caught off guard by Sessions’ family separation announcement in April.

Sessions “was the one that instituted the zero-tolerance process on the border that resulted in both people being detained and the family separation,” Kelly said. “He surprised us.”

Without singling out anyone, Kelly blasted the administration for failing to anticipate the massive public outcry sparked by the policy. A federal judge in June ordered a halt to most family separations at the border and demanded that families affected by the policy be reunified immediately.

In the interview, Kelly threw cold water on President Donald Trump’s longtime promise to build a wall along the U.S.-Mexico border.

“To be honest, it’s not a wall,” Kelly said. “The president still says ‘wall’ — oftentimes frankly he’ll say ‘barrier’ or ‘fencing,’ now he’s tended toward steel slats. But we left (the idea of) a solid concrete wall early on in the administration, when we asked people what they needed and where they needed it.”

Illegal immigration into the U.S. will only subside when our country’s “demand for drugs” decreases and economic opportunity in Central America increases, Kelly added.

Trump on Monday denied having “abandoned” his “concrete Wall” concept.

“An all concrete Wall was NEVER ABANDONED,” the president wrote on Twitter. “Some areas will be all concrete but the experts at Border Patrol prefer a Wall that is see through (thereby making it possible to see what is happening on both sides). Makes sense to me!”

An all concrete Wall was NEVER ABANDONED, as has been reported by the media. Some areas will be all concrete but the experts at Border Patrol prefer a Wall that is see through (thereby making it possible to see what is happening on both sides). Makes sense to me! — Donald J. Trump (@realDonaldTrump) December 31, 2018

Critics say Kelly, previously lauded for his no-nonsense attitude, failed to rein in Trump’s worst impulses and steadfastly advocated for some of the administration’s most controversial policies, including family separation at the border.

Kelly told the Times that critics should judge his performance in the White House based on what the president did not do while he was at his side.

Asked why he stayed in the role for 18 months, the departing chief of staff responded that “military people don’t walk away.”

Head over to the Los Angeles Times to read Kelly’s full interview.

This article has been updated to reflect Trump’s Dec. 31 tweet.