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 offices of several Democratic senators were targeted, including that of Senate Minority Leader Chuck Schumer, D-N.Y., NBC News confirmed. Two law enforcement sources briefed on the matter confirmed that “dozens of letters” were sent, but because law enforcement continues to intercept the letters, they declined to put an exact number on how many have been received or seized.

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.

Three law enforcement sources briefed on the matter told NBC News later in the day that preliminary testing of the white powder substance showed that the contents are not hazardous.

However, additional testing will continue to take place at law enforcement labs around the country to ensure the letters do not contain anything dangerous.

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 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 NBC News. The notice said no House members had been identified as intended recipients of the mailings.

The U.S. Postal Inspection Service and Capitol Police referred questions from NBC 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.

One of the letters with a suspicious substance inside was addressed to Schumer, which was intercepted by authorities, according to a spokesperson for the Democratic leader.

An office for Sen. Alex Padilla, D-Calif., was among those targeted, a spokesperson for the senator told NBC 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.”

NBC News reported Wednesday that state election 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.

Threats against election workers have increased in recent years, including incidents of suspicious substances being mailed to their offices. It happened in 2023 and just weeks before the presidential election in 2024.

CORRECTION (Oct. 15, 2026, 5:23 p.m. ET): A previous version of this article misidentified which Oklahoma office received a package. It was the state election board, not the secretary of state’s office. The article also mischaracterized election oversight in Oklahoma. That’s carried out by the election board, not the secretary of state.

Brennan Leach is an associate producer for NBC News covering the Senate.

Frank Thorp V is a coordinating producer and off-air reporter covering Congress for NBC News, managing coverage of the Senate.

Julie Tsirkin is a correspondent covering Capitol Hill and the White House.

Rebecca Shabad is a politics reporter for NBC News based in Washington.