WASHINGTON (AP) — Donald Trump’s campaign chairman Paul Manafort resigned on Friday in the wake of a campaign shake-up and revelations about his work in Ukraine.

In a statement issued as he arrived in Louisiana to tour the flood-ravaged state, Trump said Manafort offered his resignation Friday morning. The billionaire called Manafort “a true professional.”

“I am very appreciative for his great work in helping to get us where we are today, and in particular his work guiding us through the delegate and convention process,” Trump said.

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Manafort’s resignation comes a day after The Associated Press reported that confidential emails from his firm contradicted his claims that he had never lobbied on behalf of Ukrainian political figures in the US.

Emails between Manafort’s deputy, Rick Gates, also a top Trump adviser, and the lobbying firm Mercury LLC showed that Manafort’s firm directly orchestrated a covert Washington lobbying operation on behalf of Ukraine’s then-ruling political party.

The effort included not just legislative outreach but also attempts to sway American public opinion and gather political intelligence on competing lobbying efforts in the US.

Manafort and Gates never registered as foreign agents for their work as required under federal law.

Also Friday, Ukrainian anti-corruption investigators released copies of handwritten ledgers detailing possible cash payments from Ukrainian political figures to Manafort totaling more than $12 million. Details of the payments described in the ledger were first reported by The New York Times. Manafort denies receiving those payments.

Earlier this week, Trump brought in a new campaign chief executive and campaign manager following a disastrous stretch in which the New Yorker committed a series of errors and fell behind Democratic nominee Hillary Clinton in both national and battleground state preference polls.

The new campaign manager, Kellyanne Conway, had described Manafort and Gates as part of a new “core four” atop the Trump operation with her and CEO Stephen Bannon.

Bannon is moving to the Trump campaign from the top of Breitbart News, a conservative website that has emerged in recent years as a social media colossus in politics — one that has been unabashedly supportive of Trump’s campaign lately.

His installation at the top of the Trump campaign offers fresh evidence that the GOP nominee has no intention of reining in his brash, outsider’s style or cozying up to the GOP establishment despite his campaign’s recent struggles.