Czeczko was found hanged, Poland's PAP state news agency reported, citing the Nexta website, which announced the news on Twitter.
According to Belarusian authorities, the ex-soldier was found hanged in his flat in Minsk.
The investigators said they were considering all possible explanations, including a violent nature of the death, PAP reported.
Czeczko deserted from the Polish military last December, fleeing to Belarus. He was detained near the village of Tushemla, and later applied for political asylum, Belarusian officials said.
A soldier in an artillery regiment of the Polish Army’s 16th Pomeranian Mechanised Division, Czeczko was charged with desertion, and army officials said he had had run-ins with the law in the past.
After deserting to Belarus, he appeared on state media, accusing Poland of mistreating illegal migrants, PAP reported.
Czeczko went so far as to say he “was forced to kill people" while serving on the Polish-Belarusian border. It is not certain if his actions in Belarus were out of his own will, according to PAP.
Polish authorities have said Czeczko was used by Belarus in a disinformation operation against Poland.
Belarus border crisis
To stave off an influx of illegal migrants from Belarus, Poland has introduced new, stricter border protection rules and is building a 5.5-metre-high, 180-kilometre-long protective wall along its frontier with the Russian ally.
Last year, the European Union, NATO and the United States accused Belarus' strongman leader Alexander Lukashenko of orchestrating a migrant crisis at the Polish-Belarusian border in retaliation for Western sanctions against his regime.
Source: IAR, PAP