European leaders and some experts said the terrorism question just made the flow of migrants across the union’s borders more politically loaded.

The increasing likelihood that one of the >Paris attackers entered Europe with a Syrian passport last month, amid the hundreds of thousands of refugees fleeing war and persecution, has alarmed leaders and reinvigorated talk about tightening border controls.

But after months of discussion on exactly that point — and as many leaders and others warned that infiltration by would-be attackers was a grim possibility — Europe is no closer to resolving the issue, or even agreeing on what must be done to reduce that risk.

European leaders and some experts said the terrorism question just made the flow of migrants across the union’s borders more politically loaded and the burdens on border officials even more daunting.

“Security-wise, a situation that was complicated before has just gotten much more complicated,” said Marc Pierini, a visiting scholar at Carnegie Europe, a non-governmental group based in Brussels, and a former European Union ambassador to Turkey and Syria. And the need to serve both migrants and security interests burdens a system that is poorly equipped to deal with both.

“The Paris attacks have put a lot of pressure on border officials and security services in all countries, including in Austria,” said Peter Webinger, a senior official for migration and asylum in Austria’s Interior Ministry. “We really need to be extremely cautious, and I emphasise this strongly, not to mix migration process and terrorism.”

Part of the complication with border checks involves the different kinds of borders in Europe: the outer border, such as between Turkey and Greece, which marks the boundaries of the European Union, and internal open borders of the 26-nation Schengen area. Chancellor Angela Merkel of Germany has been pressing appeals to Italy, Greece and Turkey to secure Europe’s external borders and thus preserve freedom of movement in the Schengen zone.

That position is shared by President Francois Hollande of France, who on Monday, even after increasing evidence that the attackers came from both outside the European Union and from within it, emphasised that erecting walls and fences “would be the end of Europe”.

Despite mounting criticism of her open welcome of refugees, Ms. Merkel has stressed that Germany will not emulate Hungary, which fenced shut its southern borders with Serbia and Croatia. Germany has about 2,200 miles of land borders, including a 500-mile border with Austria, where refugees arrive after travelling via Turkey, Greece and the Balkans.

Her concerns about protecting Europe’s open borders have been matched by the lack of practicality in shutting them, especially in recent months, as hundreds of thousands of migrants trudged from country to country toward the heart of Europe.

That human tide has left security forces all along the trail from Greece through Macedonia, Serbia, Croatia, Slovenia and Austria with little choice but to let migrants on through to their desired destination of Germany or Scandinavia.

Checks conducted en route vary, leaving the authorities with an incomplete picture of who is arriving.

Some 5,000 refugees are arriving daily at the five crossing points between Bavaria and Austria.

The authorities try to take everyone’s fingerprints and check all identity documents, but the data is not stored because of German legal protections of privacy, Johannes Dimroth, a spokesman for the German Interior Ministry, said on Monday.

However, most of the new arrivals are applying for asylum and are taken to a registration centre, where they are formally registered before a much more thorough check and an interview — with a translator, if necessary — as the asylum application is reviewed, Dimroth said.

The countries the migrants enter before reaching Germany have even fewer protections. For Austria, which more than 500,000 people have travelled through this year, just over 70,000 requested asylum and went through a full check and registration.

The large majority move on, with no record being taken of their having been there, because the country has insufficient infrastructure to do so, Interior Ministry officials in Vienna said. Most migrants also refuse to be registered before they reach the country in which they want to settle, they added. — New York Times News Service