Two U.S. diplomatic outposts in Pakistan received envelopes containing an unknown white powder last week, sparking a security scare, a U.S. official said.

Tests are being conducted on the substance, U.S. Embassy spokesman Lou Fintor said. He declined comment Sunday on whether any embassy staffers had become ill or were under observation.

The envelopes were sent to the U.S. Embassy in Islamabad and the U.S. consulate in Karachi on Thursday and Friday. Fintor says both facilities have remained open, with no plans to close them.