"We've been told that the buildings have to go through a process in which they are cleaned with gas," one aide told Reuters. "It's impossible to work there under those conditions, and it could take a few weeks. All the buildings that tested positive for anthrax are going to be closed for some time."

Another well-placed congressional aide said he had also heard that it could take weeks before the buildings reopen. He said gas may be used to clean up the buildings, but that was not definite.

"They have to decide how they want to do the cleanup," he said.

Four buildings linked to the Capitol have tested positive for anthrax in the past week, since an anthrax-laced letter was received at Sen. Tom Daschle's offices. They are the Hart and Dirksen Senate office buildings, the Ford House office building, and an off-site Capitol police facility.

Those buildings as well as all other House and Senate office buildings remained closed for a sixth straight day this week while an environmental sweep for anthrax continued. But lawmakers returned to work in the Capitol.

A spokesman for House Speaker Dennis Hastert said it was still unclear when any of the office buildings could reopen, even the ones where anthrax has not been found.

"It's unclear. We haven't got a lot of tests back yet," spokesman John Feehery said.

"If the tests come back fine, we'll reopen those areas quickly. If they don't, we won't," he said.

The buildings where anthrax was found were declared a "warm zone" by officials last week, requiring that anyone entering them wear protective clothing.

This week evening, spokesman Dan Nichols of the Capitol Police refused to say how long the office buildings would be closed, but said that the cleanup of the anthrax-positive buildings had not even begun. He could not be reached for comment this week.