Ottawa ? The Ottawa Post Office was set to open at 1 p.m. Thursday after an anthrax scare closed the building Wednesday and halted mail delivery.

An unmarked envelope with a powdery substance in and around it prompted the closure on the one-year anniversary of the Sept. 11 terrorist attacks.

Tests on Thursday still read negative for anthrax, though final confirmation from the Kansas Department of Health and Environment won’t be given until Friday, said Barbara Conus, Franklin County Health Department director.

Officials there have given the go-ahead for the post office to resume operations Thursday afternoon.

Employees Thursday morning set up a contingency operation at the National Guard Armory in Ottawa, where they sorted and delivered mail that had accumulated Wednesday in collection boxes around town, Conus said.

A post office employee early Wednesday discovered the suspicious envelope  which was not addressed or postmarked and contained no written threat  in a nonpublic area of the building, 401 S. Hickory St.

Ottawa Police continue to investigate the origin of the envelope.