The 12 employees who handled the envelopes were being kept under medical observation.
The Daily Telegraph reports that Santander, Europe's biggest bank by market value, said it had called in Spain's Guardia Civil police and its specialist unit dealing with chemical threats as a precaution, and was sending off the envelopes to be analysed.
The Guardia Civil later revealed on Twitter that a preliminary investigation of the incident pointed to a false alarm.
The 12 employees who handled the envelopes were being kept under medical observation, though none was suffering from any unusual symptoms, the bank said.
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