Ex-Trump campaign chairman Paul Manafort’s accountant testified Friday in federal court that she had concerns about the accuracy of her client's tax returns but filed them anyway.

The testimony, on the fourth day of the trial, comes as prosecutors try to prove that Manafort knowingly violated tax and bank laws related to his political work overseas. Manafort has pleaded not guilty.

Cynthia Laporta, a certified public accountant with the firm KWC who is testifying with immunity, admitted she did have concerns about the representations given to her about loans by Manafort and his business partner, Rick Gates.

“Did you believe those representations?” Assistant U.S. Attorney Uzo Asonye asked Laporta on Friday afternoon.

“No,” she replied.

“Did you still file those taxes?” Asonye asked.

“Yes,” Laporta said.

Laporta confirmed she was testifying under immunity because of the fear that she could be prosecuted for her actions.

Laporta also told prosecutors she “very much” regrets filing Manafort’s tax returns.

“At the time did you believe it was right or wrong?” Asonye asked.

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“Wrong,” answered Laporta. “I could have refused to file ... but that could expose the firm" to the risk of litigation.

Laporta went on to say she could have called Manafort and Gates liars, but said Manafort was “a longtime client.”

The Manafort defense hinges on convincing jurors that Manafort was the boss who didn’t handle bills or accounts, but left that to Gates, who is cooperating with prosecutors and may testify as its star witness.

“Rick Gates had his hand in the cookie jar and couldn’t let his boss find out,” Manafort defense attorney Thomas Zehnle said during opening arguments.

T.S. Ellis, a 78-year-old Reagan-appointed judge in the case known for colorful comments, said this week that the prosecution cannot prove a key part of their case unless prosecutors call Gates to the stand.

Earlier Friday, Ellis reminded prosecutors of the high bar for conviction -- that they must prove the former Trump campaign chairman knowingly violated tax and bank laws.

Ellis indicated he believes the prosecution has demonstrated Manafort had control of foreign bank accounts despite checking a box on tax returns saying he didn’t.

But Ellis reminded prosecutors, “The government has to prove that [Manafort] knew what the requirement was and that he deliberately violated it.”

A source close to Manafort’s team told Fox News the defense has not yet decided whether to have Manafort testify during the trial. The source said the decision will be made after the prosecution rests its case, which could take place next week.

The trial continues Monday afternoon, when the defense will have the opportunity to cross-examine Laporta.

Fox News’ Anne Ball contributed to this report.