The FBI, U.S. Capitol Police and U.S. Postal Inspection Service are investigating letters containing suspicious substances that were mailed to “various addresses nationwide, including multiple Congressional offices,” according to a notice that senior staff in Senate offices received Thursday morning. “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 said, according to two people who received it. The notice included an image of one of the suspicious envelopes and said that the letters “may contain different variations of postage stamps.” It also warned that suspicious letters, packages or substances “should be treated as potentially hazardous and handled in accordance with established safety protocols.” The notice outlined how staff should handle the items and noted that mail and packages delivered to Senate offices or the Capitol complex are screened and tested for potential threats. A similar notice was sent to House lawmakers and staff Thursday from the House sergeant at arms. The notice said that while mail and packages delivered to the House offices in the Capitol are rigorously screened, district offices should be use safe mail handling procedures, including the use of “mail hoods” — screening equipment that allows the handling of mail without exposing staff to danger — provided by the sergeant at arms, according to two congressional offices who shared the message with Jattvibe News. The U.S. Postal Inspection Service and Capitol Police referred questions from Jattvibe News to the FBI, which confirmed in a statement that the agencies are investigating “a series of suspicious mailings sent to various addresses nationwide, including Secretary of State offices and their related election offices.” The agency did not specify the locations of the offices targeted.“Some of the letters contained an unknown substance and we are working closely with our law enforcement partners to respond to each incident,” the statement said. “We are also working with our partners to identify the individual or individuals responsible for the letters.”The notice sent to Senate staff did not say whether the congressional offices involved are the ones in Washington, D.C. or local and regional offices. Congress is on recess leading up to the midterm elections, so most lawmakers are back home in their districts as they campaign in the final weeks of the races. Congressional staff, however, are still on the Capitol campus. An office for Sen. Alex Padilla, D-Calif., was among those targeted, a spokesperson for the senator told Jattvibe News in a statement. “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. Follow up questions should be directed to the FBI.”A letter with a suspicious substance was also sent to Democratic Sen. Chris Murphy’s Hartford, Connecticut, office, his spokesperson, Josh Sorbe, said in a statement. The FBI, Capitol Police and sergeant at arms informed the office about the letter, Sorbe said, adding that the senator and his staff “were not in immediate danger.”Jattvibe News reported Wednesday that secretaries of state’s offices in Louisiana, Ohio and Oklahoma were sent packages this week containing mysterious powders. Oklahoma and Louisiana reported receiving the envelopes, while Ohio said U.S. Postal Service investigators intercepted a package destined for it.These offices oversee and manage the election process in their respective states, and these incidents come just weeks before Election Day on Nov. 3.