Republican South Carolina Sen. Lindsey Graham said Tuesday that his “deep dive” into the origins of the Trump-Russia investigation would include testimony from former Trump campaign associate George Papadopoulos, whose interactions with the mysterious Joseph Mifsud allegedly led to the FBI opening an inquiry into Trump in July 2016.

Papadopoulos was swept up in special counsel Robert Mueller’s investigation, and he ultimately pleaded guilty about lying to investigators.

Graham, the chairman of the Judiciary Committee, said his committee “will be looking at the counterintelligence investigation of the Trump campaign.”

“We will call Papadopoulos and we will find out what happened,” Graham promised.

Graham said he would wait until the completion of the investigation by Justice Department Inspector General Michael Horowitz into possible abuse of the Foreign Intelligence Surveillance Act. The allegations of abuse are related to the Justice Department and the FBI using a salacious and unverified dossier compiled by British ex-spy Christopher Steele to surveil a member of the Trump campaign.

Steele had been hired by the opposition research firm Fusion GPS, which itself had been hired by the Clinton campaign and the DNC through the Perkins Come law firm, facts which were not revealed to the FISA Court.

Trump granted Attorney General William Barr broad declassification authority, and Barr selected U.S. Attorney John Durham to be his right-hand man in a separate inquiry into why and how the Justice Department and the FBI launched an investigation into Trump.

“We’ll wait on Mr. Horowitz to do his report about the FISA warrant abuse,” Graham said. “But we’ll take a deep dive into the 2016 surveillance by the FBI and other organizations and campaigns to make sure we know what we’re doing and that there are protocols in place to ensure that we don’t play politics with the law.”

In recent days, Graham has been shedding more light on what his own follow-up investigation is going to look like, saying last week that he had questions for former FBI Director James Comey about his handling of the dossier, for former FBI Deputy Director Andrew McCabe about the importance of the dossier in the FISA applications, and for Papadopoulos about whether he was really trying to make inroads with the Russians.

Papadopoulos pleaded guilty in October 2017 to making false statements to the FBI about his contacts with Mifsud and served just 12 days in federal prison late last year. He is now on supervised release.

"I’m going to deep-dive into how this stuff started,” Graham said last weekend. “How could it go so far if there was no collusion?” Graham said that the probe by the Justice Department and the FBI into possible ties between the Trump campaign and Russia that began under President Obama was a “politicized investigation.”