Citigroup is chalking up its wayward $900 million payment to lenders of the embattled cosmetics giant Revlon to one thing: human error.

The bank — which said on Monday it has been in the process of upgrading its loan operations platform after a review it undertook last year — said it mistakenly transmitted the payments after an employee didn’t manually select the correct system options in its loan operation software.

But firms being sued to recoup the money said they haven’t been able to get the bank to explain its mistake.

The duelling court filings late Monday came after Citigroup has recovered hundreds of millions of dollars, but some of the lenders are balking at its demand to return the payments, saying that Revlon was in default on their loans and owed them the money.

A group of lenders said in a letter to a Manhattan judge that the bank has declined their offer to conditionally return the money — “in exchange for a standard indemnity Citibank would pay back the funds were it later determined that Citibank was not entitled” to them.

Administrative agents are generally tasked with collecting and distributing interest payments and providing other housekeeping services on a loan. In its filing Monday, Citigroup said the 2016 loan to Revlon allowed certain lenders to have all or a portion of their share of the loan be repurchased by Revlon.

When a loan is repurchased like this, Citigroup said employees have to manually adjust the share of the loan held by the remaining lenders. In the case of the 2016 loan to Revlon, a Citigroup employee did not manually select the correct system options — allowing the loan to be paid back in full with interest.

“Unfortunately, the manual checks of that selection also failed to detect the mistake,” Citigroup said in its court filing. “Citibank has determined that the mistaken payments resulted from human error.”

The bank said it has put “significant, additional controls in place.”

“We take pride in the role that we play as a global leader in financial services and recognise that an operational error of this nature is unacceptable,” Citigroup said in a statement.

Citigroup had been acting as an agent for Revlon’s loans, collecting funds from the company and distributing them to creditors. It said it had intended to make interest payments on Revlon’s behalf but accidentally transferred a sum more than 100 times as big from its own accounts.

The lenders said in their letter that Citigroup has yet to explain to them what happened.

“On multiple occasions, Citibank has advised the undersigned counsel that it believed it would be able to demonstrate, through a prompt factual presentation, that the disputed payments were in fact innocuous mistakes,” according to the filing. “Yet, as recently as this afternoon, Citibank advised that it was not yet prepared to shed light on or otherwise explain the putative mistake.”

Citigroup has sued roughly a dozen firms seeking to recover the funds and secured court orders last week freezing an amount totaling more than half of what was accidentally sent pending the results of its lawsuits.

Citigroup has begun briefing watchdogs, including the Office of the Comptroller of the Currency and the Federal Reserve, about how it mistakenly misdirected so much money, people familiar with the matter have said.

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