Donald Trump on Thursday ramped up criticism of the judge presiding over fraud cases against Trump University, declaring that U.S. District Judge Gonzalo Curiel has “an absolute conflict” in the case because of his “Mexican heritage.”

“I’m building a wall. It’s an inherent conflict of interest,” Trump told the Wall Street Journal, noting his policy proposals on immigration and Curiel’s background.

Trump has been hurling attacks at Curiel since February, calling him “biased” and a “hater,” as well as constantly calling the judge a “Mexican.” Curiel was born in Indiana, but is of Mexican descent.

The Republican presidential candidate also told the Wall Street Journal that he believes that Curiel is biased because he is the former colleague of an attorney representing one of the plaintiffs in a Trump University case.

But the lawyer, Jason Forge, told the Journal that he has not interacted with Curiel socially.

Curiel has not responded to Trump’s attacks. But the judge last week ordered some documents in the case unsealed, citing some of Trump’s past attacks on the court and Trump’s status as a presidential candidate.

“Defendant became the front-runner for the Republican nomination in the 2016 presidential race, and has placed the integrity of these court proceedings at issue,” Curiel wrote in the order.

The unsealed documents revealed testimony from former Trump University employees calling it a “fraudulent scheme” that “preyed upon the elderly and uneducated.” And Trump University “Playbooks” unsealed show that employees were directed to encourage people to max out their credit cards and dip into their retirement funds to pay for courses and real estate ventures.