Conservative lawyer John Yoo, a Justice Department official under President George W. Bush who authored the so-called “torture memos,” wrote in a New York Times op-ed Monday that he had “grave concerns” about President Trump’s use of executive power.

“While my robust vision of the presidency supports some of Mr. Trump’s early executive acts — presidents have the power to terminate international agreements like the Trans-Pacific Partnership, for example — others are more dubious,” Yoo said.

Yoo took issue specifically with Trump’s move to withdraw from NAFTA, as well his order to build a border wall.

“Immigration has driven Mr. Trump even deeper into the constitutional thickets,” Yoo said, in reference to the executive order banning most immigration from seven predominantly Muslim countries.

Yoo said he believed that the “bad” policy “falls within the law,” but decried former Mayor Rudy Giuliani’s comments connecting it to a Muslim ban.

“Had Mr. Trump taken advantage of the resources of the executive branch as a whole, not just a few White House advisers, he would not have rushed out an ill-conceived policy made vulnerable to judicial challenge,” Yoo said.

He also said that Trump’s firing of acting attorney general Sally Yates, whom Trump accused of betrayal because she would not defend the travel ban, raises “concerns.”

“Mr. Trump was thus on solid footing, because attorneys general have a duty to defend laws and executive orders, so long as they have a plausible legal grounding,” Yoo said. “But the White House undermined its valid use of the removal power by accusing Ms. Yates of being ‘weak on borders and very weak on illegal immigration.’ Such irrelevant ad hominem accusations suggest a misconception of the president’s authority of removal.”

Yoo noted his support of Bush administration endeavors such as the opening of Guantanamo, as well as President Obama’s use of drones and surveillance techniques in his administration.

“But even I have grave concerns about Mr. Trump’s uses of presidential power,” Yoo said.