Ravil Maganov, the chairman of Russia’s second-largest oil producer Lukoil, died on Thursday after falling from a hospital window in Moscow, two sources familiar with the situation said, becoming the latest in a series of businessmen to meet with sudden unexplained deaths.

The sources confirmed reports by several Russian media that the 67-year-old had plunged to his death, but the circumstances surrounding his fall were unclear.

Russian state news agency Tass reported the death as a suicide, citing a law enforcement source.

It quoted the source as saying Maganov had been admitted to hospital after suffering a heart attack and was also taking anti-depressants.

Reuters was unable to confirm those details.

Three sources told Reuters that, based on their close acquaintance with Maganov, they did not believe he would have killed himself.

Another source close to the company said there was a belief inside Lukoil management that he had committed suicide, but the source had not seen evidence or documents to support that.

Moscow police referred Reuters questions about the death to the state Investigative Committee, which did not respond to a request for comment. Lukoil is a private company that competes with Russian state energy giant Rosneft.

It said in a statement that Maganov had “passed away following a serious illness”. “Lukoil’s many thousands of employees mourn deeply for this grievous loss and express their sincere condolences to Ravil Maganov’s family,” it said.

At least six other Russian businessmen, most with ties to the energy industry, have died suddenly in unclear circumstances in the past few months.