(CNN) Suspicious envelopes were sent to the homes of Georgia Republican politician Karen Handel and her neighbors as well as two Atlanta-area news stations, law enforcement said.

Law enforcement responded to the suburban Atlanta neighborhood Thursday afternoon after a resident reported receiving an envelope, Roswell Police spokeswoman Lisa Holland said. The envelopes contained threatening letters and white powder.

Later Thursday, news stations WAGA Fox 5 and WXIA 11Alive received similar envelopes, the FBI said. An investigation is underway to see if they are related.

Preliminary field tests on the substance mailed to WAGA suggest the powder consisted mostly of baking soda, the FBI said.

The FBI said the letter WAGA received was very similar to those received by those in Handel's neighborhood.

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