MINNEAPOLIS (AP) — A Minneapolis apartment complex evacuated Tuesday afternoon because of concern about a possible hazardous material is being re-opened after authorities determined there was no danger to the public, a police spokesman said.

John Elder said that as a precaution police will guard overnight the one apartment where the report of a substance originated. But he said police have not yet heard back the results of testing on the substance.

"We don't know what it is," he said.

The Minneapolis Fire Department tweeted that the evacuation Tuesday involved an "unconfirmed report of ricin," a poison found naturally in castor beans.

Minneapolis police and fire crews responded around 4 p.m. to a medical situation possibly caused by a hazardous material in an apartment. Elder said first responders learned the victim had gone to a hospital, and emergency crews began evacuating the building.

A hazmat team entered the building, found the substance and took it to the Minnesota Health Laboratory for identification.

Fire crews evacuated The Marshall, an apartment complex that caters to students, and sealed off the area.

Authorities believe the possible contamination was limited to one apartment.