Senate Judiciary Committee Chairman Lindsey Graham says he is open to putting Barack Obama under oath as he probes potential White House involvement in opening a investigation into unproven ties between the 2016 Trump campaign and Russia.

Graham says one of the biggest remaining mysteries in the Trump-Russia saga that needs to be unraveled is who told President Obama about the counterintelligence investigation into then-candidate Trump. The South Carolina Republican wants answers from the former president.

As part of his probe, Graham is prepared to call former FBI Deputy Director Andrew McCabe, former FBI Director James Comey, former CIA Director John Brennan, and former Director of National Intelligence James Clapper to testify.

Graham joined Sean Hannity on Fox News on Wednesday night to say he was determined to get to the bottom of the launching of the Trump-Russia investigation, and that the “bottom line” for him was figuring out what Obama knew, when he knew it, and whom he learned it from.

“I can't imagine an investigation of the Republican nominee for president — a counterintelligence investigation of his campaign — was not approved at the highest level,” Graham said last night. “I cannot imagine it happening without somebody in the White House knowing about it."

The FBI’s probe into alleged connections between the Trump campaign and the Russian government was dubbed “Crossfire Hurricane” and officially launched on July 31, 2016, but there are questions about whether the investigation started earlier than that and whether the White House was involved.

When Hannity asked if that meant asking Obama questions under oath, Graham seemed open to the idea.

“Absolutely,” Graham replied.

Graham specifically named McCabe, Comey, Brennan, and Clapper as people he'd like to ask about whether they informed Obama about the investigation, what they told him, and why no one ever informed Trump about allegations of Russians attempting to infiltrate his campaign, with the senator pointing out that “the whole point of a counterintelligence investigation is to protect people.”

“And yes, I’d like to know what President Obama thought about the investigation,” Graham added.

The Washington Examiner reached out to Graham’s office about whether he officially planned on calling McCabe, Comey, Brennan, and Clapper to testify, and whether Obama might asked any questions too, but no response has been provided.

Graham also criticized CNN’s decision to hire McCabe as a commentator even as the fired former FBI deputy director is under criminal investigation through the Justice Department and is being heavily scrutinized by DOJ Inspector General Michael Horowitz, whose report on Foreign Intelligence Surveillance Act abuse is expected to be released in September or October. The surveillance warrant applications targeting Trump associate Carter Page required the approval of top members of the FBI and DOJ, including McCabe.

“CNN has just hired the alleged architect of a criminal conspiracy against the Trump campaign,” Graham said. “So how much time and attention do you think they are going to give to the Horowitz report when they hired the guy that Horowitz is investigating?” McCabe joined Clapper at the network.

Horowitz released a 39-page report in February 2018 detailing multiple instances where McCabe “lacked candor” with then-FBI Director James Comey and with investigators related to him greenlighting the disclosure to the media of sensitive information related to the FBI’s investigation into matters related to Hillary Clinton. McCabe was fired by then-Attorney General Jeff Sessions the day he planned to retire.

The DOJ is weighing criminal charges against McCabe.