US authorities are on high alert after dozens of suspicious letters containing white powder were discovered, addressed to members of Congress, political figures and election offices across the country.

According to information obtained by ABC News from law enforcement sources, more than 100 letters are believed to be circulating through the US postal system, triggering a major response from the FBI, US Capitol Police and the US Postal Inspection Service.

The case came to light after an advisory was sent to Senate staff on Thursday, warning of “multiple letters containing suspicious substances that have been mailed to various addresses nationwide, including several congressional offices”.

The nature of the white powder has not yet been established, while federal authorities are working to identify the person or people behind the mailings.

Chuck Schumer and other Democratic senators among the recipients

Among the political figures targeted by the suspicious letters was Chuck Schumer, the Senate Democratic Minority Leader, according to a source familiar with the case.

A letter containing suspicious material was also addressed to the district office of Democratic Senator Chris Murphy of Connecticut. However, postal authorities intercepted it before it reached its destination.

Murphy’s office confirmed the incident, explaining that the timely intervention of the relevant authorities meant there was no immediate danger to the senator or his staff.

“Thanks to these efforts and the various security protocols in place, neither the senator nor his staff were in immediate danger. Our office thanks the investigators working on this case and intends to continue working closely with them as the investigation proceeds,” the statement said.

ABC News confirmed that suspicious letters had also been sent to Democratic Senators Kirsten Gillibrand and Lisa Blunt Rochester, although neither received them.

‘The suspicious letter was intercepted in time’, Gillibrand says

New York Senator Kirsten Gillibrand confirmed in a statement that her New York office was among the targets.

“Earlier this afternoon, a suspicious piece of mail was sent to my New York office and was safely intercepted by the US Postal Inspection Service,” she said on Thursday.

“I am deeply grateful to the postal inspectors for protecting my staff,” she added.

Lisa Blunt Rochester’s office issued a similar statement, confirming that a letter containing a suspicious substance had been sent to it.

“The letter was identified by Capitol Police and handed over for investigation. Thanks to existing security protocols and the officers’ swift response, neither the senator nor her staff are in immediate danger,” the statement said.

Congress issues warning amid fears letters could reach lawmakers’ offices

The office of the Senate’s Sergeant at Arms, the official responsible for security and maintaining order in the chamber, issued a warning to staff, noting that some of the suspicious letters might still reach congressional offices.

The advisory urged staff to exercise particular caution when handling suspicious mail. One of the characteristics identified in connection with the letters was that they bore multiple postmarks.

A similar alert was sent to members of the House of Representatives by the chamber’s security office.

According to the guidance, lawmakers’ offices can forward mail received in their constituencies to the Capitol complex, where it can undergo more stringent security screening.

“The Office of the Sergeant at Arms has been made aware of reports concerning letters containing suspicious white powder substances that have been received by public officials across the United States,” the advisory stated, according to a copy obtained by ABC News.

Capitol Police referred the US network to the FBI for further information about the investigation.

FBI: Suspicious letters also sent to election offices in at least 15 states

The case extends beyond congressional offices, with suspicious mail also discovered at agencies involved in administering elections.

The FBI confirmed on Thursday that it was working with the US Postal Inspection Service to investigate “a series of suspicious mailings to various addresses nationwide, including offices of Secretaries of State and the election offices under their jurisdiction”.

According to the federal agency, the letters also contained an unknown substance that is being examined by authorities.

By Thursday evening, election offices in at least 15 US states had reported receiving suspicious mail or discovering letters addressed to them.

The FBI said efforts were under way to identify those responsible.

“We are also working with our partners to identify the individual or individuals responsible for the letters. The FBI and the US Postal Inspection Service will continue to protect the public and ensure the security of elections for the American people,” the statement said.

Possible links between the incidents remain under investigation

US authorities are investigating the origin of the letters and seeking to establish whether the incidents were part of a coordinated operation.

For now, it remains unclear whether the letters addressed to senators are connected to those recently sent to the offices of state Secretaries of State and election authorities across the country.

It is also not yet known whether the white powder discovered in the letters is genuinely hazardous or whether the mailings were intended to cause fear and disruption.

The FBI and other relevant agencies are continuing their investigation, with authorities placing particular emphasis on protecting political figures, public-sector employees and electoral processes.