The probe concerned the response to the April 10, 2010, crash of a Polish government Tupolev Tu-154M plane near Smolensk, western Russia.
All 96 people aboard were killed, including President Lech Kaczyński and his wife Maria.
The decision ends all seven strands of an investigation that was launched in 2016, after the conservative Law and Justice (PiS) party came to power.
Two strands had been discontinued in March, and the remaining five were closed on July 31.
The decision is subject to appeal by the defense minister and the head of the Committee for Investigation of National Aviation Accidents.
National Public Prosecutor’s Office spokesman Przemysław Nowak said at a news conference on Wednesday that investigators found no evidence that Polish officials deliberately harmed the country's interests in their dealings with Russia.
"The investigation, conducted for 10 years, showed that the conduct of the Polish authorities in April and May 2010 did not meet any of the legal elements of diplomatic treason," Nowak said.
"In particular, the Polish authorities did not act to the detriment of the Republic of Poland and had no intention of doing so," he added.
The offense of diplomatic treason applies when a person authorized to represent Poland in relations with a foreign government or international organization intentionally acts against the country’s interests.
One of the main allegations concerned the legal basis chosen for cooperation with Russia during the technical investigation.
Polish and Russian authorities each conducted separate criminal and aviation inquiries.
Poland agreed that the Russian-led technical investigation would operate under Annex 13 to the 1944 Chicago Convention on International Civil Aviation.
Critics, including former Defense Minister Antoni Macierewicz, alleged that the government of then-Prime Minister Donald Tusk had selected a framework that weakened Poland's position and surrendered control of the investigation to Russia.
Prosecutors rejected that claim after questioning 488 witnesses and examining extensive documentation.
Nowak said the Annex 13 procedure was the most favorable option available to Poland. It allowed Polish specialists to work at the crash site immediately, obtain access to the wreckage and evidence, and participate in Russian technical activities.
The other possible legal frameworks offered fewer guarantees and carried greater risks, he said.
Poland also continued its own criminal and aviation investigations under domestic law.
A second major strand concerned a memorandum signed on May 31, 2010, by then-Interior Minister Jerzy Miller.
The agreement provided Poland with copies of the flight recorder data and left open the possibility that Russia could transfer the original recordings after completing its investigation.
Those who reported the alleged offense argued that Miller had acted against Poland’s interests by accepting copies rather than insisting on the originals.
Prosecutors found that Polish negotiators had repeatedly demanded the original recordings, but Russian officials refused to include a binding commitment to hand them over.
Nowak said Miller’s practical choice was to sign the memorandum and immediately obtain copies, with the possibility of receiving the originals later, or refuse to sign and risk receiving nothing.
Investigators found that Miller had taken every available step to secure the most favorable agreement possible and had neither intended nor caused harm to Poland.
The remaining strands concerned an alleged delay in establishing the Polish aviation accident commission, allegedly false legal opinions and the failure to use a parliamentary ratification procedure for the agreement governing the Russian technical investigation.
Earlier allegations that Polish officials had abandoned an oral agreement with Russia to conduct a joint criminal investigation were discontinued in March.
Prosecutors also rejected claims that Poland should have requested a formal joint investigative team.
Nowak said the separate, main criminal investigation into the causes and circumstances of the crash was also "undoubtedly approaching its conclusion."
The National Public Prosecutor’s Office recently received a Polish translation of a supplementary report prepared by an international panel of experts.
The report was commissioned to provide a comprehensive assessment of the crash's causes and course, according to state news agency PAP.
The victims' relatives and their lawyers are now being given access to the translated report. They may challenge its conclusions and request additional evidence.
Nowak said further information about the experts' findings could be released within weeks.
The 2010 disaster remains one of the most politically sensitive events in modern Polish history, and its legal and political aftermath has continued to divide opinion for years.
(rt/gs)
Source: IAR, PAP