A wave of suspicious letters mailed to members of Congress is likely to continue, House officials warned Monday, describing a white powder in the envelopes.

So far, “none of the letters received by Congressional offices have been determined to contain hazardous substances,” according to a memo sent to members and staff and obtained by CQ Roll Call.

But more offices may receive similar letters “in the coming days,” House Sergeant at Arms William McFarland wrote in the memo.

At least a few dozen House Democrats have already spoken out about receiving suspicious mailings, according to a review of social media posts and statements from lawmakers. 

On Monday morning, Rep. Suhas Subramanyam, D-Va., said his district office in Leesburg was evacuated after receiving a suspicious envelope. “These acts of political intimidation have no place in our country,” he said in a statement.

Last week marked the 25th anniversary of anthrax attacks on the Capitol, and the white powder in the letters had raised fears of a repeat. 

The FBI confirmed Thursday it was working alongside the U.S. Postal Inspection Service to investigate “a series of suspicious mailings sent to various addresses nationwide, including Secretary of State offices and their related election offices.”

Photos shared by some lawmakers show “DSA” in the upper left hand corner alongside a return address once tied to the Democratic Socialists of America. The group has denied involvement, with a spokesperson telling The New York Times, “D.S.A. did not send these letters.”

Rep. Hillary Scholten, D-Mich., said in a statement Friday that her office received a letter that “made a direct and specific reference to an impending ‘civil war’ beginning after the election.”

Several lawmakers said the similarities between the envelopes had helped their staff identify them before opening them. “A member of our team identified a letter matching the description and return address, immediately secured the envelope without opening it, and called local law enforcement and US Capitol Police,” Rep. Mark DeSaulnier, D-Calif., posted Friday on social media.

While most of those posting about the mailings so far are House Democrats, several senators, including a handful of Republicans, have also described being targeted. 

“Today my Omaha office received a letter with a powdery substance. The letter matches the description of letters sent to other congressional and state offices across the nation,” Sen. Pete Ricketts, R-Neb., wrote on X Friday. 

Sens. Ted Cruz, R-Texas, Mike Rounds, R-S.D., and Susan Collins, R-Maine, similarly announced they were targets of suspicious mail sent to their state offices. A spokesperson for Collins said a letter was also addressed to her Capitol Hill office but was intercepted.

“Despite escalating political rhetoric and harassment, we will continue our work for the people of Maine,” Collins’ chief of staff said in a statement.

“This situation is deeply concerning and follows a trend of intimidating tactics that impact our team members. This has no place in our nation. America is better than this,” Rounds wrote on X on Friday. 

After the attacks in 2001, when an intern opened a letter to then-Sen. Tom Daschle and anthrax poured out, security officials made significant changes to how mail is handled at the Capitol, adding offsite screening. But lawmakers also have offices around the country in their home states and districts. Mail arriving there should be “received utilizing established safe mail handling procedures, including the use of SAA provided mail hoods,” according to the memo sent to House staff Monday.  

Some lawmakers have described the tense moments after their staff discovered powder in the envelopes, leading to hazmat responses and evacuations. 

“An envelope containing a suspicious white powder was delivered to my Santa Cruz district office and opened by staff this morning as part of her normal daily morning routine responding to constituents. Upon realizing that there was powder in the envelope, she immediately put the envelope in the garbage can, cleared fellow staff members out of the office, and called 911,” Rep. Jimmy Panetta, D-Calif., said in a Friday statement.