Her condition has not yet been reported by officials.
A fire official at the scene told The Times that they have found what they believe to be two pipe bombs and that at least one person, a poster worker, was injured.
East Chicago Fire Chief Anthony Serna said firefighters were called about a report of an explosion at the post office just before 6 p.m. Wednesday.
K-9 units from the Lake County Sheriff's Department were at the scene investigating the incident.
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He said he could not confirm if there were any other explosions but bomb technicians were preparing to sweep the post office for more explosive devices. "I can confirm there was at least one detonation".
Authorities said at an evening news conference that two employees were inside at the time of the blast and another was out making deliveries. Police and fire crews have blocked off the roads surrounding the post office on Chicago Avenue.
Agents from the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives were on the scene.
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