At least two employees of the US embassy in Paris, the French capital, are being treated for suspected poisoning after opening mail, it has been reported.
Paul Patin, a spokesman for the US embassy, told reporters at the scene that a number of staff had been taken for medical check-ups on Friday after receiving a "suspicious" letter, but gave no further information.
"We don't know if it was poison. We don't have any confirmation that they were hurt," he said.
Patin added that the letter was being analysed by chemical experts, but did not say where the correspondence came from.
Preliminary test results have indicated that the envelope was not harmful, an embassy statement said.
Earlier a Paris police official told the AP news agency that two members of staff were feeling "unwell" after opening mail and that an investigation was under way.
Other reports suggested three staff may have been involved.