Suspicious letters containing white powdery substance sent to election offices ahead of the midterms.

The letters were sent to Secretary of State's offices in Louisiana, Ohio, and Oklahoma.

NBC News reported:

Secretaries of state's offices in Ohio, Louisiana and Oklahoma were sent packages this week with mysterious powders, just weeks before the offices are set to oversee the midterm elections.

Oklahoma and Louisiana reported receiving the envelopes, while Ohio said U.S. Postal Service investigators intercepted a package destined for it.

The national groups representing secretaries of state and state election directors expressed concern about the suspicious letters in a letter Wednesday.

"As we have each stated in the past, threats against election officials and workers have no place in our democracy. Those who administer our elections should never fear for their safety while carrying out their essential duties," the National Association of Secretaries of State and the National Association of State Election Directors said in a joint statement.

"The FBI and U.S. Postal Inspection Service are investigating a series of suspicious mailings sent to various addresses nationwide, including Secretary of State offices and their related election offices. Some of the letters contained an unknown substance and we are working closely with our law enforcement partners to respond to each incident. We are also working with our partners to identify the individual or individuals responsible for the letters. This FBI will continue to safeguard the public and secure elections for American citizens."

"Election officials take these matters seriously and work closely with law enforcement and security partners to protect their personnel and facilities," the statement added.

Ohio Secretary of State Frank LaRose said these kinds of "cowardly threats will not deter us" from delivering secure and accessible elections.