California representative Nancy Pelosi’s office received “a suspicious letter” on Wednesday in what appears to be the latest incident of ill-intentioned mailings targeted at US Congress members before the midterm elections.

The offices of more than three dozen members of Congress across the political spectrum have received envelopes containing white powder in recent days. One lawmaker’s office said preliminary tests indicated the powder was not hazardous.

A spokesperson for Pelosi said the letter was “consistent with mailings reported by other congressional offices” and that everyone in the office was safe.

“Speaker Pelosi remains focused on serving the people of San Francisco and is concerned for the members and staff whose offices have experienced similar incidents,” Nicholas Martin, a spokesperson for the speaker emerita, said in a statement.

“She is grateful for the efforts of the US Capitol police, the San Francisco police department and other law enforcement agencies to thoroughly investigate this series of intimidation across America.”

More than 100 letters thought to contain white powder were reportedly thought to be circulating in the US mail system. Election offices across the US have also reported receiving suspicious mail.

The mailings are under investigation by the FBI, the Capitol police and the US Postal Inspection Service. The FBI declined to confirm “specific instances or letters”, but said in a statement earlier this week: “The FBI and US 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, and congressional offices.”