Washington — Federal investigators said Thursday that they are looking into "suspicious" letters mailed to state election officials and congressional offices, some of which contained unknown substances.

At least seven Democratic Senate offices and one House member's office were sent the letters, most of which were intercepted before they were delivered. Senate Minority Leader Chuck Schumer's office was one of those targeted, CBS News has learned. At least two Senate Republicans were also sent letters on Friday.

The FBI and U.S. Postal Inspection Service are investigating the "suspicious mailings," according to Brendan Donahue, the deputy chief inspector at the USPIS. Donahue told reporters earlier in the day that some letters have contained an unknown substance and that they were sent to "various addresses nationwide," including "secretary of state offices and the related election offices."

"We are committed to protecting our nation's election officials from threatening and dangerous mail. Any threat or attack on an election official is unacceptable," Donahue said. "Our partners, including the FBI and we at the Postal Inspection Service, will bring all of our resources to bear on these investigations."

The Senate sergeant at arms also issued a notice to congressional staff informing them of the investigation. The message, which was obtained by CBS News, said congressional offices were among those who received the suspicious letters.

A spokesperson for Delaware Sen. Lisa Blunt Rochester told CBS News that a state office received a suspicious letter that contained a white powder. The letter was not opened in the office, the spokesperson said. 

A letter meant for New York Sen. Kirsten Gillibrand was found at the senator's office in New York City and postal inspectors were on the scene to investigate, an aide for the senator said. A letter intended for one of Maryland Sen. Angela Alsobrooks' former state office was intercepted by the Postal Service, her office said Friday.

Six workers in Democratic Rep. Ritchie Torres' office in the Bronx were exposed to a white powder found in a letter on Friday. The substance was determined to be harmless. Torres said the address on the envelope indicated it was from San Antonio, Texas, and his team has turned it over to the FBI for further investigation.

A spokesman for Democratic Sen. Chris Murphy of Connecticut also said 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."

A letter was also sent to Sen. Alex Padilla's office, according to a spokesperson for the California Democrat. 

"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 spokesperson said.

A suspicious letter was sent to the office of Democratic Sen. Chris Coons of Delaware, according to a spokesperson, who said the office was "working with law enforcement."

On Friday, Republican Sens. Mike Rounds of South Dakota and Pete Ricketts of Nebraska reported receiving suspicious letters. 

Ricketts said his office in Omaha was sent a letter with a "powdery substance" that matched the description of other suspicious packages, and law enforcement "confirmed the substance was not harmful."

Rounds said his office in Sioux Falls was evacuated after it received suspicious mail. 

The House sergeant at arms sent a notice to the lower chamber's offices on Thursday stating that no House members were known to have been targeted, a message that came before the discovery of powder at Torres' office.

Federal officials are working to identify the responsible parties and have intercepted some of the letters, Donahue said. Donahue declined to provide an exact number of incidents or discuss possible connections, citing the ongoing investigation. He added that investigators would bring their full resources to bear on the cases.

The development comes less than three weeks before the midterm elections. Federal investigators also probed a series of suspicious letters ahead of the 2024 elections.

In addition to the congressional offices, top election officials in at least eight states have reported that their offices were sent suspicious packages in recent days.

Delaware's Department of Elections said in a post on X that it "received and opened a letter which was discovered to contain a white powdery substance" on Thursday. "Staff members followed security and USPS instructions on handling suspicious mail," the post said.

Louisiana Secretary of State Nancy Landry said Tuesday that an employee in her office's Elections Division opened a package and found a white powder.

"Law enforcement responded to today's incident and conducted tests that found no evidence of harmful substances," she wrote on Facebook, adding that no one was injured.

The Oklahoma State Election Board said it received an envelope containing "suspicious" white powder Tuesday morning, and its office was evacuated.

Paul Ziriax, secretary of the election board, said no one was hurt as a result of the incident, and law enforcement is investigating.

The Wisconsin Elections Commission said it received a "suspicious mail piece" on Wednesday, but "field testing confirmed no hazardous material was detected."

The secretaries of state in Pennsylvania, Maryland, New Hampshire, Ohio and Mississippi said their offices were also sent suspicious mail, but the deliveries were intercepted by the U.S. Postal Service or law enforcement before they arrived.

Jaala Brown and Alan He contributed to this report.