The West may be considering a thaw with Moscow, but Poland's new conservative government could further strain relations with the country's former Soviet overlord by reopening an investigation into the death of President Lech Kaczynski in a plane crash in Russia in 2010.
An inquiry by the previous government returned a verdict of pilot error, but the winner of Poland's October election, the Law and Justice (PiS) party led by Kaczynski's twin brother Jaroslaw, says an onboard explosion could have caused the crash.
Now in power for the first time since the tragedy, the party wants a new inquiry and, possibly, help from international courts and foreign secret services, in examining its theory.
Though PiS never definitively accused Russia of orchestrating the president's death, it has said the Kremlin benefited from the crash, which also killed the central bank chief, top army brass and many lawmakers, triggering a period of political turmoil.
PiS officials have also accused Moscow of prolonging its investigation, and withholding evidence, such as the black boxes and the plane's wreckage. Russia says these cannot be returned until its criminal probe is concluded.
"One could almost think that the Russians have something on their conscience, after all," Poland's new Foreign Minister Witold Waszczykowski told the Nasz Dziennik daily.
He has also said he cannot rule out the possibility that the president was assassinated.
"Either they solve this case amicably, or it will be necessary to hand it over to international tribunals, and challenge the investigation in the European Court of Human Rights," Waszczykowski said, adding Poland would also ask its NATO allies for help.
Challenging Moscow in international courts, as well as relaunching the inquiry, with Russia as the prime - if not openly named - suspect, is likely to damage relations between Poland and its old foe, already fragile over the Ukraine crisis.
A Russian foreign ministry official, asked about the Polish government's stance, referred to comments on Nov. 12 by a ministry spokeswoman, who said the tragedy should not be considered politically.
"We should act in accordance with procedure and provide all-round assistance to each other," she said. Russia had been acting openly, cooperating with partners, and assisting Polish investigators, she said.
Reopening the case could also test Poland's ties with its NATO allies, who may shy away from controversy as some try to re-engage with Moscow to fight Islamic State and forge a peace deal in Syria.
Domestically, the move may be popular among Poland's conservative electorate, deeply distrustful of Russia, but it will also reopen memories of a case whose handling by the government aroused wide disagreements among Poles.
The country's worst such disaster since World War Two, the crash took place near Smolensk, western Russia, close to the place where Stalin's secret police shot some of the 22,000 Polish officers and intellectuals it executed in 1940. For decades, Moscow blamed Nazi Germany for the mass executions.
The massacre is an enduring symbol for Poland of its suffering at Soviet hands, and president Lech Kaczynski had been flying in to commemorate it.
For Jaroslaw, whose strong relationship with his twin brother was a defining aspect of their political ascent together, winning power may be an opportunity to settle a personal score, critics say.
He has long accused then prime minister Donald Tusk, who is now head of the European Council of heads of EU states, of being indirectly responsible for the crash, caused, in his view, at least partially by the government's negligence.
An investigation by Tusk's government produced no evidence of that.
Already, the PiS choice for defense minister - Antoni Macierewicz - suggests that Smolensk will be high on the agenda.
Macierewicz, who in opposition opened his own unofficial investigation into the crash, has been the leading proponent of theories that something more sinister than pilot error was behind the crash.
"The government headed by (Russia's then prime minister Vladimir) Putin is fully responsible for this tragedy," he told the European Parliament in March.
He has told army cadets that finding the truth about the crash is the biggest challenge facing the Polish army.
During his investigation, Macierewicz relied on evidence provided by experts who used props such as crushed drinks cans and burst sausages to explain that the Russian-made aircraft had exploded in mid-air before crashing.
A file photo shows Jaroslaw Kaczynski, twin brother of Polish President Lech Kaczynski, praying in front coffin of brother at airport in Warsaw. Photo: Reuters
The experts had not visited the crash site, and used photographs from the internet. Their meetings sometimes verged on the farcical, with proceedings at one point interrupted by a hoax Skype call claiming to be from Russian President Vladimir Putin.
Earlier this month, Polish President Andrzej Duda expressed support for Macierewicz's experts, saying the official conclusions of Polish and Russian inquiries did not "hold up" when confronted with evidence.
The new government has now shut down the official website devoted to the state investigation, which refuted claims that the plane was brought down by an explosion.
Political analysts say reopening the investigation, and Macierewicz's anti-Russian rhetoric, mostly serve domestic political goals, as they are likely to appeal to core voters.
Opinion polls show only one in five Poles believes the cause of the crash has been properly explained. Nearly a third accept to some degree the suggestion that the president may have been assassinated.
Speaking shortly after the October election, Kaczynski told supporters gathered to commemorate his brother's death that more would be done.
"I'm convinced that the conditions for reaching the truth have now been established."
Despite the foreign minister's announcements, it was not immediately clear whether the government would actually ask Poland's allies for help in investigating the crash. Shortly after the election, PiS leader Kaczynski said a proper Polish inquiry would suffice.
A senior source close to the party leadership said the new government realized that, at a time when some Western countries want to rebuild ties with Moscow, prioritizing the case could complicate Poland's relations with its allies.
Poland was therefore likely to stick to well-established ways of challenging Moscow, such as international tribunal lawsuits over the withholding of evidence, technically Polish property, the source said, speaking on condition of anonymity.
But Poland was likely to insist that its Western intelligence partners, who want Warsaw to join the fight against Islamic extremists, reciprocate by handing over any intelligence that may be relevant to the crash, he added.