Mr. Brodsky was arrested in early June upon his arrival in Poland because of a European arrest warrant issued by Germany and Germany is now seeking his extradition, the spokesman said, declining to be named in line with department policy.

The spokesman had no estimate of how long it could take for Mr. Brodsky to be extradited, saying "the matter is now in the hands of the Polish authorities.''

If a person agrees to an extradition, it's a matter of a few days, he said. "But here this seems unlikely.''

The Polish Interior Ministry declined to answer any questions on the arrest.

Police in the United Arab Emirates said the elaborate hit squad linked to the Jan. 19 slaying of Mr. al-Mabhouh in Dubai—one of the founders of Hamas's military wing—involved some 25 suspects, most of them carrying fake passports from European nations. Dubai's police chief, Lt. Gen. Dahi Khalfan Tamim, has said he is nearly "100 percent'' certain that Mossad, Israel's spy agency, masterminded the killing.

German news weekly Der Spiegel reported that the arrest in Poland has already led to some diplomatic friction. The Israeli embassy has urged Polish authorities not to extradite Mr. Brodsky, the magazine reports in its issue to be published Monday.

Germany's Foreign Ministry had no comment on the case and referred to an ongoing judicial investigation by the federal prosecutor's office. The country's top investigating unit deals with all cases affecting internal or external security, including terrorism or espionage.

After a German passport was used by a person linked to the Dubai slaying, the prosecutor's office in February started investigating a possible connection to a foreign intelligence agency.

Authorities in the western city of Cologne had issued a passport to a man named Michael Bodenheimer. A man using that name was among the assassins who killed the Hamas operative, according to Dubai police.