The Russian authorities have attributed the death six months ago of Aleksei A. Navalny, the country’s main opposition leader, to a grab bag of diseases aggravated by heart arrhythmia, a conclusion described as preposterous by his widow on Thursday.

Mr. Navalny’s widow, Yulia Navalnaya, speaking in a recorded statement posted online, said she received a three-page document from Russia’s Investigative Committee last week saying there was no cause to open a criminal investigation into her husband’s death in a remote Arctic prison because a combination of medical factors had killed him.

“Here’s a man under constant surveillance in prison, a globally recognized politician, the leader of the opposition, and suddenly he dies within an hour from a ‘combined illness,’” Ms. Navalnaya said. “The report is a lie, and they are hiding what happened that day.”

She demanded that the authorities release the full medical report and return Mr. Navalny’s personal effects to his family, including his notebooks and the cross that he wore. The family has requested this material repeatedly, she said, but was told it would have to wait until the committee, the main agency for investigating possible federal crimes in Russia, completed its report.

Ms. Navalnaya said that she believed the real cause of death was murder by the state, and that in the absence of a credible official inquiry, which was unlikely as long as President Vladimir V. Putin remained in power, the family would conduct its own investigation, no matter how long it took. The Kremlin has strongly denied that it had Mr. Navalny killed.