Brunon Kwiecień was the first Pole to be accused of terrorism.
The cause of his death is to be determined by a post-mortem on Thursday, PAP reported.
In an interview with public broadcaster Polish Radio on Wednesday, Polish Deputy Justice Minister Michał Wójcik said that a preliminary examination had found that Kwiecień had died from natural causes.
“There were no third parties involved, nor was it a suicide,” Wójcik added, citing preliminary findings.
In December 2015, Kwiecień was convicted of planning a terrorist attack on a Polish state institution in 2012, of attempting to incite two students to carry out the attack, of illegal possession of firearms and trafficking in firearms. He was then sentenced to 13 years in jail.
In April 2017, a court reduced his sentence to nine years.
Kwiecień, who worked at the Kraków Agricultural University in southern Poland, had allegedly planned to ram a vehicle packed with explosives into the lower house of parliament when both the president, then Bronisław Komorowski, and the prime minister, at the time Donald Tusk, were expected to be present.
Kwiecień admitted to planning an attack but claimed he had been encouraged by an agent working for Poland's Internal Security Agency (ABW) in a sting operation.
Source: PAP/Polskie Radio