Attorney Robert Costello, former legal adviser to onetime Trump "fixer"-turned-foe Michael Cohen, told Fox News his ex-client is a liar and unreliable, while saying New York County District Attorney Alvin Bragg does "not want to get to the truth" in his prosecution of former President Donald Trump.

Costello appeared before the grand jury Monday in Bragg's investigation into Trump, testifying that Cohen is a "serial liar." Later on "Tucker Carlson Tonight," Costello called the case "weak to say the least," adding his two-hour sitdown in lower Manhattan gave him the feeling the district attorney was uninterested in the truth.

"I called them up after I saw Michael Cohen on TV stating things that he said he was going to tell the grand jury… that were contrary to what he told us when we first represented him in April 2018. So I'm sitting at home watching these lies, and I said, I've got to do something about it," Costello said.

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"I don't represent Donald Trump, but I do stand for justice. And I think I have a legal obligation to inform both sides."

When the U.S. Attorney's office phoned Costello to talk about his time representing Cohen, he asked if they had documentation of Cohen waiving his attorney-client privilege.

"And they said, you presume correctly. I said, fine, send it over, I'll be delighted to talk to you. That waiver is very clear. And once we had that, I prepared 330 emails, a bunch of text messages…" Costello said, adding he also provided documents related to an interview from the then-Democratic-led House Intelligence Committee investigators whom he said included current Rep. Daniel Goldman, D-N.Y., then the director of investigations while Rep. Adam Schiff, D-Calif., was the chairman.

Costello said he then sent that same documentation to Trump's legal team and Bragg's office, adding he further requested to meet with the Democrat personally.

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Michael Cohen, attorney for The Trump Organization, arrives at Trump Tower in New York City, U.S. January 17, 2017. REUTERS/Stephanie Keith - RTSVX0Z

Michael Cohen at Trump Tower on 5 Av (REUTERS)

"I wanted him to look me in the eye and assess my credibility," he said. "Did I have more credibility than Michael Cohen? Because Michael Cohen, in my opinion, was lying about just about everything."

Costello claimed he once met with an emotionally distraught Cohen, who allegedly said, "I swear to God, Bob, I don't have anything on Donald Trump."

Costello said his client continued acting incredibly distraught, and purportedly said, "I will do whatever the ‘f’ I have to do, I will never spend one day in jail."

"Now what he's saying is, I'll lie, cheat, steal, shoot somebody. I will not spend a day in jail," Costello said to host Tucker Carlson. "Do you think a guy whose mentality is that is going to not admit that he has information on Donald Trump? But having said that, the DA's office didn't ask me questions to bring that up, and I brought it up anyway."

"I ignored their questions and simply gave them the information," he added.

In 2018, Cohen was convicted of making false statements to a federally-insured financial institution, campaign finance violations and tax evasion and sentenced to three years at a white collar federal prison in Otisville, N.Y.

Costello later said he told Bragg's team he once held a position in the Southern District of New York, and that he "wouldn't touch a witness like Michael Cohen for any amount of money – you simply cannot rely upon this guy."

The attorney then referenced an interview Cohen did moments earlier on MSNBC, during which he said Cohen did not recall waiving attorney-client privilege – leading Costello to hold up a document he claimed was that waiver.

In Cohen's interview with MSNBC, the onetime Trump counsel conversely slammed Costello over similar public comments made following his afternoon appearance in New York.

"So, very much like most of everything that came out of his mouth, he lacks any sense of veracity here," Cohen said regarding a claim he would be returning to testify again Wednesday.

"I didn’t know Bob Costello from a hole in the wall," Cohen later added while claiming another partner in the firm connected the two and that Costello "was never my lawyer."

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Cohen said he didn't recall waiving privilege, saying that if in fact he did, "I'd like to know when, how and where."

He also said he was indeed "incredibly distraught" in the referenced meeting with Costello, saying it was in part because until that time he had only had one run-in with law enforcement in his life: a 1985 speeding ticket while driving back to college.

In a statement to Fox News Digital, responding to Costello's comments made during his press conference, Cohen said: "IF Bob Costello's comments were any more fantastical, he would be a bestselling fiction author." 

"It is important to note, I never slept with Stormy, I stated years ago that the payment was done at the direction of, in coordination with and for the benefit of Donald," Cohen told Fox News Digital. "Truth is truth and the documents in the possession of the DANY demonstrate this." 

Fox News' Danielle Wallace and Marta Dhanis contributed to this report.