President Trump has ordered the FBI to conduct an investigation into his Supreme Court nominee, Brett Kavanaugh, a process that will be "extensive," said retired FBI agent Warren Flagg. The FBI will "go out and conduct these interviews in a thorough manner as quickly as possible and they will report – no opinion – they will report that to the people eon the committee," Flagg told CBS News' Reena Ninan.

Mr. Trump ordered the FBI investigation after Kavanaugh and Dr. Christine Blasey Ford, the woman who accused him of sexually assaulting him when they were in high school, testified before the Senate Judiciary Committee on Thursday. On Saturday, Mr. Trump said he is confident the FBI is doing a "great job."

The FBI has "free reign," Mr. Trump said. "They will do what they have to do -- things we haven't thought of and hopefully at the conclusion everything will be fine."

Flagg noted on "CBS Evening News" that Kavanaugh has gone through six FBI background checks, each one more "extensive" than the one before. Kavanaugh told the Senate panel that he had gone through six background checks, "six separate FBI background investigations over 26 years."

"This one, though, has to do with additional evidence that was presented at the hearing – people were named, a diary was used, there were other people who have come forward, other people who have made allegations – all those people will be interviewed as soon as possible," Flagg said.

CBS News has learned that as part of the investigation, the FBI has reached to Deborah Ramirez, the second woman who accused Kavanaugh of sexual assault. Ramirez has claimed that Kavanaugh exposed himself to her while they were both students at Yale University in the 1980s

Kavanaugh denies the allegations. He submitted a calendar into evidence that he was not with Ford the night that the alleged incident took place.

Flagg said the polygraph that Ford said she took will not be admissible. "It can't be used in a criminal case, it's an investigative tool," Flagg said.