Polish authorities were investigating Monday whether three Pakistanis and a Ukrainian detained in Warsaw might have terrorist links after finding marked-up maps in one of the men's apartments.

Police were also looking into the possibility that the case involves human smuggling, spokesman Pawel Chojecki told The Associated Press.

Poland's government, an important U.S. ally in Iraq, has put security services on alert and urged citizens to be vigilant after the Madrid train bombings (search), which killed 202 people. Poland commands 9,500 peacekeepers in Iraq, including 2,400 of its own.

The men were taken into custody Sunday after Warsaw police spotted one of the Pakistanis behaving suspiciously near the capital's main train station and asked for his documents, police spokesman Marcin Szyndler said.

The man said the documents were at his apartment. When police went there, they encountered two other Pakistanis and a Ukrainian man, Szyndler said.

A search of the premises turned up tourist maps of Poland with wide circles marked around large parts of the city, including an area with the capital's only synagogue and an area housing many embassies, Szyndler said.

He emphasized that no individual sites were marked.

"At this point there are no grounds to say that without a doubt we are dealing with terrorists," Szyndler said.

Still, Polish authorities contacted police in several countries and the European police agency Europol and were checking all items found on the men to determine "who they are and what they are doing in Poland," Chojecki said.