European countries have threatened sanctions on Russia over the alleged nerve agent attack on opposition leader Alexei Navalny, but a concerted decision will be tough to reach given the interests at stake.
Western governments are united -- at least in outrage -- over the latest in a long line of assassination attempts against critics of President Vladimir Putin.
And the Navalny case also raises particular concerns as German doctors say it involved the use of the Novichok nerve agent.
The EU last week condemned the use of a chemical weapon as "a serious breach of international law" and warned Moscow it may respond with "appropriate actions, including through restrictive measures".
A diplomat explained that "restrictive measures" is code for sanctions against individuals who would then be banned from travelling to the EU and would have any assets they hold in the bloc frozen.
There is already a precedent for such measures: in 2019, the EU added four members of Russia's GRU military intelligence service to its sanctions list.
This followed the attack on Russian ex-double agent Sergei Skripal and his daughter, who were poisoned in March 2018 in the English town of Salisbury with Novichok -- like Navalny now.
But the Skripal case was different in one crucial respect: The attack took place on the soil of an EU and NATO member, whereas Navalny was poisoned in Russia.
NATO Secretary General Jens Stoltenberg has said an "international response" is needed but has refused to speculate on what form it might take.
Steven Blockmans, an expert on EU external relations law at the Brussels-based Centre for European Policy Studies, said another key difference was that after Salisbury, British authorities soon found GRU agents had been in the UK.
"In the Navalny case the forensic evidence of an involvement of Russian intelligence services is not there. The smoking gun is still missing," he told AFP.
If Russian intelligence was involved in the poisoning, that could lead to individual sanctions, Blockmans said -- but given it happened in Russia, this will be difficult to prove.
Sanctions against individuals have to be legally watertight, since they can be challenged in court, and "conjecture" is not enough to justify them, Blockmans said.
The mere fact that Novichok was used is not enough, even though the poison was developed by the Soviet military and is not freely available.
The EU and NATO have both called for an independent international investigation into the Navalny case.
"The ball has been hit back to Moscow, saying 'show us an independent investigation, otherwise we'll take your inaction as an admission of guilt'," said Olivier Dorgans, a sanctions expert with the Hughes Hubbard & Reed law firm.
Another possibility could be broader economic sanctions, which can be more political and require less legal justification.
After the shooting down of flight MH17 in the Ukrainian conflict in 2014, the EU imposed a whole series of such measures against Russia, directed against state banks, the import and export of armaments, and the oil and gas industry.
An EU diplomat noted that the bloc's statement last week "did not explicitly mention economic sanctions but nor did it explicitly rule them out."
But Blockmans said this was unlikely, since they would need unanimous approval by all 27 member states, and some are already chafing at the impact existing measures are having on their own companies who want to do business with Moscow.
"In this case, I would assume there would not be any political support from countries like Italy or Hungary because of closer economic and political relations with Russia. Cyprus might be an outlier as well," he said.
The Navalny case has also brought calls for an end to the Nord Stream 2 project, a 10 billion-euro ($11.7 billion) pipeline which is set to double Russian natural gas shipments to Germany.
The project has been delayed for months after Washington moved to impose sanctions on companies involved, over fears of growing Russian influence.
Germany is now facing calls to abandon it altogether.