Vanguard Group, one of the largest investors in Boeing, has raised concerns over the handling of the grounded 737 Max.

It’s being reported by the Financial Times that its fund managers have become “very concerned” by reports of oversight failures following the aircraft’s two crashes, which took place in March.

According to the report, the issues raised by Vanguard – which holds a 7.1% stake in Boeing – were included in a letter from the mutual fund group’s top portfolio company engagement official.

In the letter, it reads that Vanguard had “engaged with Boeing leadership and directors to convey our concerns” and would continue the discussions.

On May 17, Boeing has said it completed development of the updated software for the 737 Max, along with associated simulator testing and the company’s engineering test flight.

The update was said to address the issues which attributed to the grounding of 737 Max jets.