Polish authorities have arrested a suspected Israeli agent in connection with the murder of a Hamas operative in Dubai in January.
Officials say the agent was arrested in early June. Media reports named him as Uri Brodsky.
Germany is seeking his extradition over a forged German passport used by one of the killers, the prosecutors say.
Mahmoud al-Mabhouh, one of the founders of Hamas's military wing, was found dead in a Dubai hotel on 20 January.
Dubai police have said they are 99% sure Israeli agents were involved, but Israel says there is no proof.
On Saturday, German prosecutors said the agent had been arrested on a warrant issued by Germany, as he arrived in Poland.
"It's now up to the Poles to decide if they are going to hand him over," a spokesman for the federal German prosecutor's office told AP news agency.
Israel's foreign ministry confirmed that an Israeli citizen had been arrested in Poland but gave no futher details.
A spokeswoman for Polish prosecutors told the Associated Press news agency that a suspect, identified as Uri B, had been arrested on 4 June in connection with the Dubai murder.
The German magazine Der Spiegel reports that the Israeli embassy in Warsaw has urged the Polish authorities not to extradite Mr Brodsky.
Forged passports from Britain, the Irish Republic, France, Australia, and Germany were used in the Dubai operation, leading to diplomatic rows between those countries and Israel.
The UK and Australia have expelled Israeli nationals over the forgeries.
Israel maintains that Mabhouh was in Dubai to procure rockets for Hamas to fire into Israel from Gaza.