FBI probes suspicious white powder letters sent to Dem senators as threat spreads nationwide

Two senators were the intended recipients of mail that federal investigators say contained "suspicious substances."

Sens. Chris Murphy, D-Conn., and Alex Padilla, D-Calif., were both informed by that their respective offices were sent a suspicious letter on Thursday following warnings in both chambers of Congress that federal investigators were "investigating multiple letters containing suspicious substances that were mailed to various addresses nationwide, including multiple Congressional offices."

Josh Sorbe, a spokesperson for Murphy, told Fox News Digital that the senators office "was sent a letter with a suspicious substance."

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"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," Sorbe said. "Due to those efforts and various safety protocols, the Senator and staff were not in immediate danger. Our office is grateful for the investigators involved in this incident, and we intend to continue close coordination as the investigation continues."

A spokesperson for Padilla told Fox News Digital, "We received the notification from the FBI that one of our offices was mailed a letter containing a suspicious substance, but it was intercepted by USPS."

The two incidents follow warnings sent out to top-ranking staff on the Hill earlier Thursday by both the Senate and House sergeants at arms that the FBI, U.S. Postal Inspection Service, and U.S. Capitol Police were actively investigating the matter, per notices obtained by Fox News Digital. 

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"Law enforcement officials are working to intercept any additional letters before they are delivered, however, it is possible that some letters may reach their destinations," the notice from the read.

The exact nature of the suspicious substances, and just how many congressional offices are intended recipients, remains unclear. 

Top staff in the House were warned by the House Sergeant at Arms that the office had "been made aware of reports of letters containing suspicious white powder substances received by public officials throughout the United States." 

"Although have been identified as intended recipients at this time, the SAA continues to monitor the situation and coordinate with law enforcement partners," the notice read. "All offices are reminded to remain vigilant and follow safe mail handling procedures to help protect staff from potentially harmful materials."

An FBI spokesperson declined to confirm specific instances or letters, but said in a statement that, "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," the spokesperson said. "We are also working with our partners to identify the individual or individuals responsible for the letters. This FBI and the U.S. Postal Inspection Service will continue to safeguard the public and secure elections for American citizens."

Included in the Senate Sergeant at Arms' notice was an image of one of the envelopes that included a return address in the top-