Cleanup crews have sealed the building to prevent any spores from escaping and are preparing to fill the building with germ-killing gas.
"We're not in a rush, we're going to do it right. It could be a month or more," postal vice president Azeezaly Jaffer said Tuesday.
Spokesman Jerry Kreienkamp added that the facility is "not going to reopen until it's proven that it's completely safe to do so."
Now that the building is sealed, plans call for it to be filled with chlorine dioxide, the same gas used to sanitize the smaller Hart Senate Office Building earlier.
Jaffer said the post office plans to do everything at Brentwood that was done at the Hart building "and probably more."
The building has been sealed, the contaminated materials have been removed and the building will now be sanitized and reinspected, he said.
Plans call for chlorine dioxide gas to be made at the site and pumped into the building to circulate through its heating and air conditioning system, reaching every part of the building.
Close watch will be kept to make sure there are no leaks and the gas will be kept there for 12 hours to kill the spores. It then can be removed and destroyed.
After the treatment, thousands of tests will be performed inside the building to make sure no live anthrax remains.
A few questions are still to be resolved, such as whether to treat the whole building at one time or do it in sections.
The postal distribution center in Trenton, N.J., also remains closed because of anthrax contamination.
"Once the tests come back successful at Brentwood we'll begin preparations at Trenton," Jaffer said.
The Brentwood facility was closed Oct. 21 because of anthrax contamination. Mail is now being handled by other offices in nearby Maryland. Mail in the facility when it was closed was taken to plants in Ohio and New Jersey were it was decontaminated with irradiation before delivery.
Five people died of anthrax after contaminated letters were mailed from New Jersey in September and October, including two Washington postal workers. More than a dozen people were infected in New York, New Jersey, Florida and Washington.
The Hart building was shut down after an anthrax-containing letter was opened Oct. 15 in the office of Senate Majority Leader Thomas Daschle.
The building was decontaminated with chlorine dioxide gas and workers are now back in their offices there.
The FBI and Postal Service are offering a $2.5 million reward for information leading to whoever mailed the contaminated letters.