WASHINGTON — Aides to Sen. John Cornyn were stuck in their offices at midday Thursday after an intern opened an envelope containing a suspicious powder. Capitol police were testing to see if it was anything dangerous.

“It’s some sort of a powdery substance,” said Sgt. Kimberly Schneider. “The hazmat team is testing the substance.”

False alarms and scares are not uncommon at the Capitol. No one was evacuated, though everyone in the vicinity was asked to remain in place, presumably to avoid the spread of any potential contaminant. Access to part of Mr. Cornyn’s office suite was blocked pending the outcome of the tests.

“One of our interns opened a suspicious envelope. Capitol police responded. Everyone is working as usual. I’m not evacuated. Sitting at my desk,” said Cornyn press secretary Brian Walsh. “The system for screening mail, since the anthrax, is very vigorous.”

Mr. Cornyn’s suite spreads over two floors of the Hart Senate Office Building, and only one floor was affected, with aides and visitors allowed to come and go on the other floor. Mr. Cornyn was in the Capitol, voting on the Senate floor, at the time, Mr. Walsh said.

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