"At this stage, there is no indication that the man's death was caused by the actions or negligence of another person," Piotr Antoni Skiba, a spokesman for the Warsaw District Prosecutor's Office, said.
Police and prosecutors have been examining the apartment in Warsaw's Mokotów district since around 1 a.m., Skiba said.
An autopsy was scheduled for Wednesday afternoon, along with toxicological and other specialist tests.
Vlasenko's medical records were found at the apartment, Skiba said.
According to the independent Russian news outlet Sota, Vlasenko served as a member of local authorities in Russia's Far East in the 1980s. He later lived in Anapa in southern Russia.
Vlasenko left Russia for Poland after the full-scale invasion of Ukraine began, initially on a tourist visa, and later obtained political asylum, Sota reported.
Vlasenko was associated with the Congress of People's Deputies, an opposition organization made up of former Russian lawmakers.
The congress was initiated by former Russian lawmaker Ilya Ponomarev, who lives in Ukraine.
The organization says it ultimately aims to become a transitional parliament for Russia.
Russia's Prosecutor General's Office designated the Congress of People's Deputies an "undesirable organization" in April 2023.
The death comes amid heightened concerns over suspected Russian operations targeting dissidents and people inconvenient to the Kremlin.