The Obama administration’s Customs director criticized President Donald Trump’s proposal for building a wall on the U.S.-Mexico border as he was leaving office.

Gil Kerlikowske told ABC News in an interview conducted shortly before leaving office on Friday that he did not “think it was feasible” nor “the smartest way to use taxpayer money on infrastructure.”

“I think that anyone who’s been familiar with the southwest border and the terrain…kind of recognizes that building a wall along the entire southwest border is probably not going to work,” Kerlikowske said.

The president made building a wall a staple of his campaign, and famously promised to “have Mexico pay for that wall.”

But Kerlikowske pushed back on Trump’s promise the wall could be done inexpensively.

“When we look at the cost — and we have about 600 miles of fencing now — we look at the maintenance and the upkeep, we know how incredibly difficult it is,” he said.

Kerlikowske said supporters of the wall ignore the real issue that immigrants can come up to points of entry, which are open for commerce.

“People can come up to those ports of entry, as they are doing now, and turn themselves in and ask for whatever laws they feel will protect them,” he said.