The person or persons who brought on the extensive investigation of President Donald Trump and his campaign will be found out, his attorney Rudy Giuliani said Monday while responding to Attorney General William Barr's summary of special counsel Robert Mueller's report.

"You're going to find out, believe me, who it was," Giuliani told Fox News' "Fox & Friends." "I can't give you a hint, [but we're] working on it. I have got some theories and a lot more than theories."

That person or persons will be held accountable under the current Department of Justice and under Attorney General William Barr, said Giuliani, because "we're in an era of a real Justice Department where just because you have a D after your name, Democrat, you know, or Clinton or something like that, you still can be held to account."

The probe was thorough and carried out by "people who had a bias to get him, people who didn't want him elected, people who were crying when Hillary Clinton lost," Giuliani said, adding that Democrats were "shameless" for their reaction.

He added he and Trump are very good friends, and it was "horrible" to see him go through the investigation.

Giuliani said Trump's legal team did not know what was in the report until they saw it, but he thinks it is "about as well-written a legal document as I have ever seen." Still, he thinks Mueller deserved "every bit of criticism he got."

"The fact that he couldn't find evidence is a testament to the fact that he is an honest man," Giuliani said. "But boy, did he try and boy did he have some people that did things that as a prosecutor for almost 20 years shocked me."