WARSAW, Poland — Polish prosecutors said Monday they have summoned European Council President Donald Tusk to testify as a witness in their investigation of a 2010 plane crash that killed President Lech Kaczynski and 95 others.

It is the second time that Tusk, who was Poland’s prime minister at the time of the crash in Russia, has been summoned by prosecutors to the country, which is currently ruled by his fierce political opponents. Spokeswoman Ewa Bialik said that Tusk was summoned for July 5.

The ruling party is led by Jaroslaw Kaczynski, the twin brother of the late president. He is suggesting that Tusk has some responsibility for the president’s death, and is also seeking to discredit Tusk, who enjoys large popularity in Poland and could threaten his party’s position.

In March, Defense Minister Antoni Macierewicz notified prosecutors he suspects Tusk of being guilty of “diplomatic treason” against Poland in 2010. He said Tusk failed to secure Poland’s participation in the investigation of the crash and the return of the wreckage from Russia, thus preventing the full clarification of its causes.

Aviation experts have found the crash to be an accident, but prosecutors are investigating whether anyone bears responsibility, for example due to neglect.

Last month Tusk testified as a witness over allegations of secret illegal contacts between Polish and Russian intelligence officials while he was government head. He was greeted in Warsaw by a cheering crowd of supporters, but also by opponents. He was questioned for eight hours and later said the case is of a political character.