President Trump’s proposed wall along the U.S.-Mexico border cannot be built with only the executive order he signed Wednesday and its construction will require congressional approval, border experts and former federal officials said. While Trump can start the wall by shifting around existing federal funds, he will need Congress to appropriate the $20 billion — and perhaps significantly more — required to complete the massive structure, the experts and former officials said.

“How is he going to fund it? You need money!” Rand Beers, a former acting Department of Homeland Security secretary in the Obama administration, said Wednesday. “He’s got to have the money. And you can’t reprogram all that money without congressional authorization.”

While congressional Republicans have said they plan to fund the wall, administration officials have said they are still discussing specific funding options with GOP lawmakers.