Uber investors Mitch and Freada Kapor want to change two things: Uber's toxic workplace and the deafening silence from Uber's backers when it comes to the "inexcusable behavior" of the company's leadership.

A former engineer's claims of sexism at Uber have rocked the company for days, leading to a teary apology from its CEO Travis Kalanick. However, they were apparently only the tip of a much deeper problem inside the company's culture. A bombshell report from the New York Times said a manager had been fired for groping women's breasts, employees had done cocaine in the bathrooms during company retreats, and a director had yelled gay slurs during meetings.

In an open letter to Uber's investors and board, the Kapors blasted Uber for ignoring the work some of its investors have tried to do behind the scenes for years to change the company culture. Freada Kapor lead a workshop on unconscious bias in 2015, she said. They've both been contacted by multiple senior officials at Uber --although "notably" never by Travis Kalanick himself, the Kapors said.

The Kapors, who invested early in Uber, are known in Silicon Valley for both their investing prowess but also their passion and engagement around issues of diversity in the tech industry. They created the Kapor Center to help increase access to STEM education and bring more diverse entrepreneurs into the tech ecosystem.

"We are speaking up now because we are disappointed and frustrated; we feel we have hit a dead end in trying to influence the company quietly from the inside," the Kapors wrote.

Specifically, the investors take issue with who Uber has chosen to lead what it calls an "independent" investigation into Fowler's claims. Former attorney general Eric Holder previously worked on Uber's behalf to advocate for the company's concerns. Arianna Huffington is on the board of the company, and the Chief Human Resources Officer reports to the executive team. All are in on the review.

"We are disappointed to see that Uber has selected a team of insiders to investigate its destructive culture and make recommendations for change. To us, this decision is yet another example of Uber's continued unwillingness to be open, transparent, and direct," the Kapors wrote.

"We intend to be thorough, impartial and objective, and we are conducting this review with the highest degree of integrity and professionalism," Eric Holder and his law partner Tammy Albarran, said in a statement.

The Kapors remain fearful that Uber will once again be able to "manage its way past this crisis and then go back to business as usual." They had tried to involve the company in diversity projects run by Kapor Capital, but failed. Now that their work inside the company has failed to bring about change, the Kapors are hoping public pressure might turn the company around.

"We are speaking out publicly, because we believe Uber's investors and board will rightly be judged by their action or inaction," the Kapors wrote. "We hope our actions will help hold Uber leadership accountable, since it seems all other mechanisms have failed.

This post originally appeared on Business Insider.