Technocrats in Brussels will readily say that what is now keeping them up at night is Spain. They are trying to see beyond the tools that so far have kept a true crisis at bay: the two rounds of low-cost loans that the European Central Bank extended to commercial banks late last year and earlier this one, and the €780 billion bailout fund.

One potential new tool, according to Mr. Deo, would be for Europe to guarantee the bank deposits of at-risk countries like Spain. This would be similar to the way the U.S. government increased deposit insurance during the financial crisis in 2008 to head off a bank run. It would be an expensive undertaking, to be sure, and one that would have to be bankrolled largely by parsimonious Germany.

But such a drastic step might steel the shaky nerves of Spanish depositors.

Just such a step was briefly considered by European policy makers last year. But it was shelved on the assumption that North European taxpayers would not be inclined to back the banking system in Spain — or in Italy, whose own banks have still not regained a solid footing, or in other euro zone convalescents.

And without an allocation of new money, there could be no new guarantees for depositors. The total banking deposits in Spain, Italy, Portugal and Ireland are €5.5 trillion, or seven times the size of the main European rescue vehicle, the European Financial Stability Facility.

The other problem is that a deposit guarantee does little good if the citizens of the country in question become convinced that their nation might soon abandon the euro for another currency — as seems to be the case in Greece, where more than €60 billion in deposits have fled banks since the crisis began. Those account holders fear having euros in the bank that could overnight become drachmas with half the value or less.

So far in Spain, there has been little sign of mass flight of deposits, perhaps, in part, because no one is seriously talking right now about a Spanish exit from the euro. What has been happening, though, Spanish bankers say, is that deposits have been moving from riskier savings banks like Bankia to safer institutions like Santander and BBVA, both of which benefit from having substantial international operations. Bankia is deemed to be so close to the brink that Spain’s government has seized control of it.

But there is no question that the Spanish problem with bad loans is growing worse by the month. Last week, official statistics disclosed that nonperforming loans through March were 8.37 percent of the total loans — the highest level since 1994, long before the adoption of the euro.