NPR's chief executive admitted Tuesday that he was aware of another, previously undisclosed harassment complaint in 2015 against the organization's top editor, Michael Oreskes, but declined to remove him from his job.

The executive, Jarl Mohn, also said he was taking a medical leave of absence and would be out at least four weeks.

Mohn's revelation of another complaint against Oreskes, who was forced to resign last week, appears to deepen the questions surrounding Mohn's handling of the matter and his continued tenure leading the public broadcaster.

In a staff-wide meeting Friday, Mohn was pelted with angry comments about his failure to act in the face of multiple allegations that Oreskes had acted inappropriately toward women for years, including during his time running NPR's newsroom. Mohn conceded that he had moved too slowly to address the problem.

In a memo on Tuesday, Mohn wrote that he had received an employee complaint about Oreskes in the fall of 2015. It was the second complaint that reached the top of the organization around that time. But Mohn previously acknowledged receiving only one such complaint. Mohn didn't spell out the nature of the newly revealed accusation but wrote that Oreskes had been "disciplined" at the time.

This means that NPR's leaders were aware of at least four complaints against Oreskes when the Washington Post reported last week on two alleged harassment incidents involving Oreskes in the late 1990s when he was the Washington bureau chief for the New York Times. The Post story triggered Oreskes's forced resignation from NPR a day later.

Mohn wrote in his memo that he didn't mention the 2015 complaint until now because NPR had promised the employee who complained "that it would be kept confidential, and it had not been publicly reported."

The statement is curious because formal harassment complaints are typically confidential. Mohn did not explain why the confidential nature of this complaint was especially important and why it didn't prompt Oreskes's removal when at least two subsequent allegations surfaced in October 2016 and last month.

He also disclosed that Oreskes had "inappropriate expenses" while employed at NPR. Although Mohn didn't detail the nature of the expenses, he said that "concerns were raised" and that NPR investigated. Oreskes recently reimbursed NPR for $1,800 in "invalid expenses," Mohn said.

"In retrospect, I did not see the bigger pattern of poor judgment and unacceptable behavior," Mohn wrote, referring to Oreskes. "I am sorry, and I have learned from this."

NPR's board plans to hire a law firm to investigate Mohn's handling of Oreskes. The board's chairman, Roger Lamay, did not respond to a request for comment.

Since last Tuesday, at least five NPR employees have come forward with new accusations against Oreskes.

Mohn said his leave of absence was related to health problems he experienced earlier this year. He said that in March, he suffered "a nearly fatal ruptured aorta" and was permitted to return to work as long as his blood pressure remained at an acceptable level. "Regretfully, the hypertension has returned to a dangerous level, and I have been instructed to take medical leave until my health returns to normal," he said.

In Mohn's absence, chief operating officer Loren Mayor will be NPR's top executive.

Mayor thus becomes the sixth person to head NPR on a permanent or interim basis since 2009. Mohn, a veteran media executive and successful investor, is in the midst of a five-year contract that ends in 2019.