The Trump administration is considering a review of U.S. interrogation methods to bring back torture and secret CIA “black site” prisons like the ones in which operatives tortured detainees under the Bush administration.

But had President Barack Obama prosecuted top-level Bush administration officials who authorized those activities, President Donald Trump might have a harder time resuming them, a legal scholar said.

Obama took action early in his presidency to close the secret overseas sites and ban torture, but he stopped short of prosecuting top-level Bush administration officials who authorized the activities. Just before his inauguration in 2009, he said, “We need to look forward as opposed to looking backwards” on torture.

“But he should have looked forward further — to a time when another president might have a very different attitude toward torture,” said Elizabeth Goitein, co-director of the Liberty and National Security Program at New York University’s Brennan Center for Justice. 

“For any type of crime, one of the key purposes of prosecution is deterrence,” she said.

And while Trump appears to be sold on the end justifying the means, “he may yet encounter… resistance from at least some officials in his administration,” Goitein said, “as well as from the rank and file of the agencies and from powerful members of Congress such as Senator John McCain.”

Trump said in an interview with ABC News on Wednesday that he “absolutely” believes torture is effective, but he said he would rely on the opinions of his Defense secretary, Gen. James Mattis, and his CIA director, Mike Pompeo, both of whom have expressed opposition to torture.

“And if they don’t want to do, that’s fine. If they do want to, then I will work toward that end,” Trump said. “But do I feel it works? Absolutely, I feel it works.”

In 2008, candidate Obama set himself from his opponent Hillary Clinton by taking the harder line on the prosecution of Bush administration for authorizing torture. He said he wanted to avoid a “partisan witch hunt,” but that if high officials “knowingly, consciously broke existing laws” and covered it up, “nobody [is] above the law.”

Obama took a different route. He quickly put a stop to torture, but stopped there. Attorney General Eric Holder began a Department of Justice investigation in 2009, and closed it in 2012 with no charges filed.

When Dianne Feinstein’s Senate Intelligence Committee, spurred by revelations that the CIA spied on its members, took on a more aggressive torture investigation in 2010, the Obama administration actively impeded its progress. 

Still, investigations were enough to make it clear that many high-level Bush administration officials violated the Constitution and the Geneva Conventions, broke U.S. law, and engaged in a cover-up. The people responsible have since moved on to high-status jobs in the legal, political, and educational systems.

It was within Obama’s power as president to authorize prosecution of the officials, but he chose not to. Goitein said that though convictions were never guaranteed, “there were certainly cases where sufficient evidence did exist to go forward.”

Trump’s move to reinstate torture is in line with his campaign rhetoric. In a question-and-answer session during the Republican primaries last February, he said, “Torture works… But they asked me the question, ‘What do you think of waterboarding?’ Absolutely fine. But we should go much stronger than waterboarding.”