An apartment complex evacuated because of concern about a possible hazardous material is being re-opened and police will guard overnight the one apartment where the report originated.

Minneapolis police spokesman John Elder says residents were allowed back in after it was determined that there is no danger to the public. But he said police have not yet heard back on the testing of a substance, adding: “We don’t know what it is.”

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

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