President Lech Kaczynski was among 96 people who died in the crash in Smolensk, western Russia in 2010. Some Poles, including the governing PiS party led by Kaczynski's twin brother Jaroslaw, believe it was not an accident.
Poland's President Andrzej Duda on Tuesday demanded that Russia hand over the wreckage of a 2010 presidential plane crash that has stoked friction with Moscow and divisions at home as Warsaw marked eight years since the disaster.
President Lech Kaczynski and his wife Maria were among the 96 people who died in the crash in Smolensk, western Russia, on April 10, 2010.
"The plane wreckage is probably the most important evidence in this case – it's still in Russia and the Russian authorities don't want to hand it over," Duda told Polish media.
"One can only ask why the Russians don't want to return the wreck," he added, after laying floral tributes on the graves of Kaczynski and his wife in the southern Polish city of Krakow.
Warsaw has repeatedly asked Moscow to hand over the wreckage and black boxes, but each time Russia has said it will only do so when its own inquiry is finished.
Ties have been strained since the crash as well as Moscow's 2014 annexation of Crimea from Ukraine, which prompted the EU to impose sanctions against Russia.
The Kremlin responded with tit-for-tat sanctions notably targeting Polish agricultural products.
The plane crash occurred as the Polish delegation was heading to a ceremony in Russia's Katyn forest in memory of thousands of Polish army officers killed by Soviet secret police in 1940 – a massacre the Kremlin denied until 1990.
Poland's governing rightwing Law and Justice (PiS) party, led by Kaczynski's twin brother Jaroslaw, has long insisted the crash was no accident.
The previous liberal government blamed bad weather and errors by the Polish pilots and two Russian air traffic controllers.
In 2015, Polish prosecutors charged the former with "being directly responsible for having endangered air traffic" and the latter with "unintentionally causing an air traffic disaster."
Having rejected the conclusions of the earlier investigation as a cover-up, the PiS government launched its own probe using new investigators.
They suggested last year that an explosion was likely to have caused the aircraft to break up in the air.
Polish justice officials have also exhumed the remains of the victims to test for traces of explosives or combustion.
Some 57 percent of Poles believe the crash was an accident, while 20 percent think it was an attack, according to a Pollster survey published on Tuesday by the Super Express tabloid daily. Twenty-three percent have no opinion.
EU President Donald Tusk, who served as Poland's prime minister at the time of the crash, is due in Warsaw later this month for questioning by prosecutors.
The liberal Tusk, who is an archrival of PiS leader Jaroslaw Kaczynski, alleged he was being politically targeted by investigators following a marathon eight hours of questioning in last August.
Senior PiS politicians have accused him of "diplomatic treason" in connection with the previous investigation.