Former Trump campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE reportedly wrote “WTF” and “this is a disaster” in an April 2015 email to Richard Gates, his longtime business associate, regarding a tax bill that was higher than Manafort expected.

Prosecutors showed the email exchange to jurors on Tuesday, according to CNN, during the sixth day of the criminal trial against Manafort. Gates reportedly testified that the email demonstrated Manafort’s frustration at having to pay those taxes.

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“How could I be blindsided like this,” Manafort wrote in the email, CNN reported. “You told me you were on top of this. We need to discuss options.”

Gates took the stand for the second straight day Tuesday, testifying that Manafort, his former boss, had him wire money directly from Cyprus-based accounts to pay U.S. vendors. Gates said he created fake invoices for those transactions.

Gates also testified Tuesday that Manafort faced financial challenges in 2015 and 2016, according to CNN. Gates reportedly said vendors reached out to him and Manafort because “loans had not been paid” and that DMP, Manafort’s consulting group, had no clients by the end of 2015.

The trial is considered the first courtroom test for special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s Russia investigation. Gates pleaded guilty in February as part of a plea agreement with Mueller’s team and is considered the prosecution’s star witness in the Manafort trial.

Prosecutors say Manafort hid millions of dollars from the IRS stemming from work he did consulting and lobbying for pro-Russia officials in Ukraine, hiding the money in overseas accounts.

Manafort’s defense team will have the opportunity to cross-examine Gates on Tuesday and is expected to focus on funds that Gates admitted stealing from Manafort.