WASHINGTON (AP) — Federal authorities are investigating letters with “suspicious substances” that have been mailed to congressional and election offices nationwide.

Authorities are urging recipients to treat the letters as “potentially hazardous.”

The Senate was informed of the investigation by a message from the Senate Sergeant at Arms that was obtained Thursday by The Associated Press.

The mailings come just three weeks before Election Day and are reminiscent of a similar threat in 2023, when several election offices received envelopes containing the powerful opioid fentanyl.

Mailed packages containing a powdery substance led to the evacuation this week of election offices in at least two states, Louisiana and Oklahoma. The Ohio secretary of state said the Postal Service intercepted a package containing a white powder intended for its office and noted that similar mailings had been sent to election officials in multiple states.

The National Association of Secretaries of State and the National Association of State Election Directors issued a joint statement condemning what they said were threats against election staff.

“Those who administer our elections should never fear for their safety while carrying out their essential duties,” they said in their statement.