Four people were taken to an Everett hospital for evaluation before being treated and released, Lake Stevens Police Chief Randy Celori said. Reported symptoms included nausea, wooziness and breathing difficulties.
"It is undetermined what was the cause," Celori said.
Crews trained in handling hazardous materials were called to search the building but no noxious chemicals were found, Celori said.
Fire crews deemed the building safe about 1 p.m.
"We had to rule out anything serious, and that's what they did," Lake Stevens Fire Battalion Chief Bruce Huston said.
Huston described the employees' symptoms as flu-like.
The incident began about 11 a.m. at the U.S. Postal Service annex at 12216 36th St. NE.
The initial call was about a female worker who reportedly became ill, Celori said. About 30 employees were screened by emergency crews.
Postal Service inspectors joined police and fire crews at the scene.
"Since the initial incident, there have been no other complaints," Celori said.
Fire and hazardous materials personnel from throughout Snohomish County assisted with the response.
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