According to reports, the family of Alexei Navalny, a critic of Putin who died in a Russian prison, was told that his body would not be released for two weeks.
His mother was told that it was being held for "chemical analysis," said a representative of Navalny.
Russian authorities have not yet confirmed the whereabouts of the body, and efforts to find it have been repeatedly shut down.
The wives of the deceased Russian opposition leaders accused them of hiding this.
In a paragraph on Monday, Yulia Navarnaya vowed to continue fighting for "Free Russia" in which she directly accused Russia of killing her husband. She also claimed that his body was kept until traces of poisoning by the nerve agent Novichok disappeared.
Navalnaya's voice sometimes trembles with sadness and anger, and she asks the audience to stand with her and "share the anger and hatred for those who dare to strangle our future."
His mother and lawyer traveled to a remote colony as soon as the news of his death broke.
Attempts to find the body were repeatedly closed by prison morgues and local authorities.
On Monday, the Kremlin said the investigation into Navalny's death was ongoing and that it was "inconclusive" at the moment.
Later, Navalny's spokesman, Kira Yamish, said investigators told Navalny's mother, Lyudmila, that they would not hand over the body within two weeks when they conducted a "chemical analysis."
British Foreign Secretary Lord Cameron also said he expects the UK and other wealthy G7 countries to impose new sanctions on any Russians involved in the deaths.
Kremlin spokesman Dmitry Peskov said that the comments of Western politicians on Navalny's death were "arrogant" and "unacceptable."
Russian prison authorities said over the weekend that Navalny was suffering from "sudden death syndrome".