Federal investigators are racing to track down the source of a wave of suspicious letters -- several containing a mysterious white powder -- that have landed in congressional offices and state election offices across more than 30 states in the final stretch before next month's midterms.

The FBI, the U.S. Postal Inspection Service and Capitol Police are jointly investigating the mailings, which a law enforcement source told reporters now number more than 100 letters suspected of carrying hazardous substances moving through the mail system, according to CBS News. Most have been intercepted by postal screening facilities before ever reaching their intended targets.

No injuries have been reported, and in at least six confirmed instances, lab testing found the powder to be non-hazardous, authorities said. Testing on other letters is continuing.

A Coast-to-Coast Scare

Election offices in Louisiana and Oklahoma were evacuated after suspicious packages arrived, and Ohio's secretary of state said the U.S. Postal Service intercepted a powder-filled envelope addressed to that office before it was opened, according to PBS NewsHour. Investigators have also flagged common physical features among many of the envelopes, including processing through a single North Texas mail facility and a recurring postmark reading "Thinking of You."

More than three dozen members of Congress have received the letters, and the list of targets crosses party lines -- several Republican senators are among those who say they received mailings, according to NBC News. Secretary of state offices and their affiliated election divisions have been hit in parallel, though officials have not confirmed whether the two tracks of mailings are connected.

MORE THAN 100 LETTERS SUSPECTED OF CONTAINING WHITE POWDER ARE NOW CIRCULATING THROUGH THE U.S. MAIL SYSTEM

The timing has drawn particular scrutiny. With early voting already underway in many states and Election Day three weeks out, the mailings have landed directly on the desks of the officials responsible for running the vote, prompting several states to tighten mail-screening protocols at election offices.

What's Next

The Postal Inspection Service says most of the material is being caught before reaching recipients, but officials have declined to say whether they've identified a suspect or a motive. Capitol Police have stepped up screening of incoming congressional mail, and lawmakers in both parties have called for a fuller public accounting once the investigation allows it.

For now, federal authorities are urging congressional and election staff to treat all unexpected mail with caution and report anything unusual immediately, rather than opening it.