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Alik Keplicz, Associated Press
A Polish Army soldier marches past the memorial with names of the 96 victims of the presidential plane crash, during a ceremony marking the fifth anniversary of the accident that killed all on board, among them President Lech Kaczynski, in Warsaw, Poland, Friday, April 10, 2015.

WARSAW, Poland — Poland is marking the fifth anniversary of the plane crash that killed President Lech Kaczynski and 95 members of his presidential entourage on a visit to Russia, the worst catastrophe to strike the Polish state since World War II and an event that deepened historical strains between the two Slavic neighbors.

Although the disaster briefly united the nation's 38 million people in profound shock and grief, it has since deepened divisions within Polish society.

There are some, including the president's surviving twin brother Jaroslaw Kaczynski, who believe that the president was assassinated by Russian authorities. They are deeply at odds with the majority of Poles, who trust a state investigation which laid a large share of the blame on pilots who chose to land in heavy fog. State investigators also put some blame on Russian air traffic controllers for giving faulty guidance.

The national bitterness was revived this week with the leak of transcripts from the cockpit shortly before the plane crashed near Smolensk. It added to evidence that an air force general was in the cockpit pressuring the pilots to make the dangerous landing — further undermining the theory of an assassination.

The president's daughter, Marta Kaczynska, who also lost her mother in the disaster, placed flowers early Friday at her parents' grave at Wawel Cathedral in Krakow.

In Warsaw, President Bronislaw Komorowski, Prime Minister Ewa Kopacz and other leaders attended a memorial ceremony at the Powazki military cemetery, gathering next to a memorial shaped like airplane wings entering the ground.

Earlier, Parliament Speaker Radek Sikorski and other parliamentary leaders placed wreaths in the building to remember the 18 lawmakers who were also killed in the crash on April 10, 2010.

Many other commemorations were scheduled throughout the day, including a memorial march to be led by Jaroslaw Kaczynski.

The crash deepened tensions with Russia. Poles are angry that the wreckage still has not been returned to Poland., Russian authorities say Poland has failed to hand over evidence needed in their investigation, but Polish officials say they are doing their best to cooperate.