The FBI and US Postal Service are investigating a series of suspicious mailings to state election officials across the country, some of which contain unknown substances, officials told reporters on Thursday.

The packages have been sent to secretary of state offices and election locations around the country in recent days, and officials are working to identify who is behind the mailings and whether they are connected, Brendan Donahue, deputy chief postal inspector, said during a press briefing.

“This remains a matter that is under investigation,” Donahue said. “This is a series of suspicious mailings.”The probes by postal authorities and the Federal Bureau of Investigation are proceeding as USPS prepares to process and deliver what could amount to more than 100 million mailed ballots during this year’s midterm elections, officials said during a briefing on broader efforts to ensure a smooth voting process.

Questions about the processing and delivery of mail ballots was heightened in recent months as President Donald Trump’s administration attempted to implement a new rule that would have required the Postal Service to restrict mail voting. However, the Supreme Court issued an order last month preventing the rule from taking effect for this year’s midterms.

Postmaster General David Steiner confirmed on Thursday the new rule will not be implemented, saying the Postal Service is “absolutely confident and ready” to deliver all ballots this year.

“We are all systems go for next month’s midterms general election,” Steiner said. “We share a common goal: instilling confidence in all Americans that their election mail will be handled securely and delivered reliably.”The Postal Service will begin using special measures starting on Oct. 24 that include extra deliveries and collections, special pickups, sorting mail at processing facilities that expedite delivery to board of elections offices and expanded hours of operations.