* Casino shares close down 9.4%

* Naouri questioned by police

* Casino has a large debt burden

PARIS, June 1 (Reuters) - Jean-Charles Naouri, the boss of heavily indebted French supermarket group Casino, is being questioned by police as part of an ongoing investigation into alleged stock price manipulation, corruption and insider trading at the company, France's financial prosecutor's office said on Thursday.

Shares in Casino, already under pressure from the company's financial problems, closed 9.4% lower.

The national financial prosecutor said in March that it had opened a preliminary investigation in February 2020 into Casino over alleged price manipulation, corruption and insider trading charges in 2018 and 2019.

When contacted by Reuters on Thursday, Casino said it had no comment to make. Naouri is the 74-year-old chairman and CEO of the company, which owns the Franprix and Monoprix chains.

The prosecutor's office declined to comment on whether the probe targeted Naouri directly.

The probe was launched in 2020 after the prosecutor's office was alerted by France's AMF market watchdog, a source close to the matter said.

Last week, Casino said it was officially starting court-backed negotiations with creditors as the company seeks a way out of its financial woes while weighing two tie-up bids from wealthy investors.

Casino has been plagued for years by hefty debt following a string of acquisitions and by declining revenues and loss of market share in an increasingly competitive domestic market. (Reporting by Benoit Van Overstraeten, Silvia Aloisi, Dominique Vidalon, Tassilo Hummel and Sudip Kar-Gupta Editing by Susan Fenton)