The FBI and the US Postal Inspection Service are investigating letters sent to state officials and election offices across the country containing an “unknown substance”.

Just weeks before November’s US midterm elections, election officers in Louisiana and Oklahoma received envelopes containing a powdery substance, leading to evacuations. A similar package containing white powder was also sent to the Ohio secretary of state, but the US Postal Service intercepted the package.

The US Senate sergeant at arms, who serves as chief law enforcement for the Senate, informed members of the upper chamber on Thursday of the investigation in a message, obtained by the Associated Press.

The notice said law enforcement officials were working to intercept additional mail, but it is possible some could reach their destination. Federal authorities have urged recipients to treat the mail as “potentially hazardous”.

Brendan Donahue, a senior official with the US Postal Inspection Service, confirmed the investigation into the suspicious mailings during a press conference on Thursday. “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.

The US Postal Inspection Service referred the Guardian to the FBI.

In a statement, the FBI said it would not confirm specific instances but said that it was “investigating a series of suspicious mailings sent to various addresses nationwide, including Secretary of State office and their related election officers” alongside the US Postal Inspection Service.

“Some of the letters contained an unknown substance and we are working closely with our law enforcement partners to respond to each incident,” the FBI said. “We are also working with our partners to identify the individual or individuals responsible for the letters.”

The FBI declined to comment on what the purported substance may have been.

In Oklahoma, state troopers determined the substance mailed to the Oklahoma State Capitol complex was non-hazardous.

In Cook county, Illinois, an envelope containing only white powder was sent to Maria Pappas, the treasurer, on Thursday morning, prompting a response from local emergency responders and law enforcement.

The powder turned out to be baking powder, Pappas’s office confirmed to the Guardian.

“Unfortunately, one-tenth of 1% of the population causes 100% of the problems: crimes, threats, murders, beatings and abusiveness, etc. Too bad that normal folk have to have their guard up when a minority of folk lash out with their craziness,” Pappas said in a statement.

Another letter containing an unknown substance was sent to the Pennsylvania state department but intercepted by the USPS, Votebeat reported Thursday.

Two organizations representing secretaries of state and state election directors released a joint statement raising alarm at the mail containing the unknown substance.

The National Association of Secretaries of State (NASS) and National Association of State Election Directors (NASED) “are deeply concerned by these incidents”, the organizations wrote in a statement on Wednesday.

“As we have each stated in the past, threats against election officials and workers have no place in our democracy,” they wrote. “Those who administer our elections should never fear for their safety while carrying out their essential duties.”

Other officials have received mail containing suspicious substances, including Chris Murphy, the Democratic senator from Connecticut. The letter was sent to his Hartford office, according to NBC.

Alex Padilla, the senator from California, was also the target of a letter containing a suspicious substance, but it was intercepted by the USPS.

Similar incidents have arisen during election cycles in the country. During the 2024 election, at least 14 election offices across the country had received mail containing white powder. In some cases, the substance turned out to be flour.

In 2023, election offices in six states also received mail containing white powder. In some cases, the powder contained trace amounts of fentanyl, a powerful opioid.

The Associated Press contributed reporting