Federal investigators are probing reports of ‘suspicious substances’ mailed to congressional offices and election offices in several US states, officials confirmed Thursday.

The threats, which both the FBI and US Postal Service are investigating, have sparked fresh security concerns in the final weeks before the midterm elections. 

The FBI said that some of the letters 'contained an unknown substance' and that it was working to identify those responsible. 

At least one Senate office was targeted, according to an update from the US Capitol Police and Senate sergeant at arms, which notified the other congressional offices about the threat.

Election officials in multiple states - including Louisiana, Oklahoma, and Ohio - were also targeted by letters - prompting widespread evacuations, or interceptions Postal Service investigators. 

In Oklahoma, the state election board's office was evacuated after receiving an envelope that contained the suspicious white powder - prompting a heavy emergency response.

And Ohio Secretary of State Frank LaRose said investigators blocked the delivery of a letter containing a similar, 'powdery white' substance to his office one day earlier.

'Election officials should never have to fear for their personal safety while performing their duties, and there is no place in a peaceful democracy for violence, threats or acts of intimidation,' he said. 

The National Association of Secretaries of State and the National Association of State Election Directors urged calm in a separate joint statement.

There are ‘security measures and contingency plans in place’ to ‘respond to potential threats and emergencies while maintaining essential election operations,' they added. 

'Those who administer our elections should never fear for their safety while carrying out their essential duties.'

The threats come as US election officials and federal agencies prepare for a higher threat environment in the run-up to the November midterms - the result of deeper political divisions and a more complex threat environment that have contributed to an uptick in harassment and attacks in recent years.

A Princeton University analysis of threats and harassment from the last two national elections found that election officials and poll workers were targeted more than any other group.

'Although election workers were able to ensure that the election remained safe and secure for voters, the data shows that they did so under intense pressure,' the researchers found. 

The analysis recommended more support and resources for state and local election officials to protect against threats and retain employees.