The FBI confirmed this evening that it is helping investigate a series of suspicious letters, some containing an “unknown substance,” that arrived at at least six sheriff’s departments across Oregon on Monday, NBC News reports.

According to the East Oregonian, Grant County Sheriff Glenn Palmer immediately broke out in a rash and was hospitalized after interacting with powder found in one of the envelopes, which are believed to be linked. From KATU:

U.S. Postal Inspector Jeremy Leder said it is “very likely” the letters are related. He said since letters sent within the state can have different dates of delivery, more letters may arrive at their locations Wednesday. He also said there is no reason to believe the general public is at risk.

At least 10 government buildings were reportedly evacuated today in response to the letters.

“It would appear that just about all counties are involved,” said an Association of Oregon Counties spokesperson.

[Image via KGW]