Investigators refused to name the suspect, but a lawyer for Vera Trifonova's family said a doctor of a public hospital who briefly treated her was charged with criminal negligence over her death.
The decision sparked cries of foul play from Trifonova's family. Her son, Vyacheslav Raikov, said prosecutors were trying to deflect blame from the system by making the doctor a scapegoat.
Trifonova, 53, a real estate executive, was accused of involvement in a scam to sell a senator's seat in the Federation Council. She was ordered to pretrial detention despite suffering from a grave form of type-2 diabetes.
Her family says her death was caused by prison authorities withholding medical care for months, possibly to force a confession.
Trifonova's death came shortly after the high-profile case of Hermitage Capital lawyer Sergei Magnitsky, who died in pretrial detention in 2009 after being held on charges that his supporters called revenge from Interior Ministry officials whom he had accused of embezzlement.
Magnitsky's death led to a public outcry that prompted Medvedev to fire about 20 senior prison officials, including the head of Matrosskaya Tishina, the Moscow pretrial detention facility where both he and Trifonova died.
The judge who ordered Trifonova's arrest was later dismissed from office, but no one in either case has been sentenced.