Federal law enforcement agencies are leading an investigation into letters with 'suspicious substances' that have been mailed to several congressional and election offices nationwide.

The FBI, US Postal Inspection Service and US Capitol Police are running the probe, which involves materials that agents are urging recipients to treat as 'potentially hazardous.'

The investigation was first disclosed in a message from the Senate Sergeant at Arms obtained Thursday. 

The notice, sent to Senate offices, said that law enforcement officials were working to intercept any additional letters, but 'it is possible that some letters may reach their destinations.' 

'Some of the letters contained an unknown substance and we are working closely with our law enforcement partners to respond to each incident,' the FBI told The Daily Mail in a statement.

'We are also working with our partners to identify the individual or individuals responsible for the letters.' 

While the FBI did not confirm any specific letters or instances, several Democrat members of the Senate confirmed receiving things.

The offices of both Democrat Senators from Delaware - Lisa Blunt Rochester and Chris Coons - said they were sent suspicious letters. 

Senator Alex Padilla of California said the Postal Service intercepted a suspicious package before it reached his office. 

And Senator Kirsten Gillibrand said postal inspectors intercepted a suspicious piece of mail addressed to her New York City office. 

News Nation reported that Senator Chris Murphy of Connecticut was also targeted, with a spokesperson saying his office was 'sent a letter with a suspicious substance.'

'Our office was promptly informed of the letter by FBI, SAA, and Capitol Police, and we continue to work closely with them and federal law enforcement,' said Josh Sorbe, a spokesman for Murphy. 

'Due to those efforts and various safety protocols, the Senator and staff were not in immediate danger.' 

In Congress, the threat appeared limited to the Senate, though the House was monitoring the situation, according to a person familiar with the matter who was not authorized to discuss the investigation publicly.

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. 

Numerous states, from Idaho to Florida, said their election offices either received letters or had them intercepted by the Postal Service.

The Wisconsin Elections Commission said in a statement that it received a letter and the white powder enclosed in it was not hazardous, according to tests. 

'Let this be very clear: this incident will not shake us,' Wisconsin elections administrator Meagan Wolfe said.

News of the letters comes just three weeks before Election Day and is reminiscent of a similar threat in 2023, when election offices in six states received mailings that contained a white powder. 

In some cases, the incidents disrupted work around local elections.

Some of those letters contained threats and traces of the powerful opioid fentanyl, leading some election offices to stock up on the overdose reversal medication naloxone.

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.

Deputy Postal Inspector Brendan Donahue told reporters Thursday that 'any threat or attack on election officials is unacceptable' and that 'we are also working with our partners on identifying the individual or individuals responsible for the letters.'