A Russian businesswoman who died in jail while awaiting trial was killed by a tube that surgeons had left in her body after an operation, investigators said Friday.

Real estate firm owner Vera Trifonova died in April after four months in jail awaiting trial on fraud charges. The 53-year-old suffered from severe diabetes and chronic kidney failure. She was nearly blind and needed to use a wheelchair during her last months in jail.

Her death strengthened a whirlwind of criticism of prison negligence, coming months after the death of Sergei Magnitsky, a 37-year-old lawyer whose pancreatitis was allowed to go untreated by prison officials.

Natalya Leontyeva, a spokeswoman for Moscow's Investigative Committee, told The Associated Press on Friday that a catheter left in a vein in Trifonova's hip caused a fatal blood clot. She refused to say if Trifonova underwent surgery before her detention or during it.

Later, a committee statement said officials were attempting to identify the doctor responsible.

Days before Trifonova's death, the judge had refused a third appeal for her release on medical grounds. The Supreme Court is considering disqualifying Judge Olga Makarova, who resigned after Trifonova's death.

Magnitsky was arrested on tax-evasion charges related to his work on a case involving allegations of police corruption.

His death caused public uproar and prompted President Dmirty Medvedev to fire more than 20 prison officials and launch prison reforms. None of the officials who Magnitsky said were to blame for his unbearable conditions has been charged.