A federal investigation of letters containing unknown substances that were mailed to elected officials’ offices has expanded as some Republicans have been identified as targets of the mailings, a person with direct knowledge of the investigation told NBC News on Friday.

The names of the GOP lawmakers have not been revealed, though they are members of the Senate, the source said. Those lawmakers join a growing list of identified targets including at least 25 state election offices and at least 10 offices of congressional Democrats.

Six of those Democrats are senators, including that of Senate Majority Leader Chuck Schumer of New York. On Friday, Reps. Seth Moulton of Massachusetts, Maxwell Frost of Florida, Ritchie Torres of New York and Bonnie Watson Coleman of New Jersey also said offices in their home states received such letters.

Democratic Sens. Chris Murphy of Connecticut, Alex Padilla of California, Lisa Blunt Rochester and Chris Coons of Delaware and Kirsten Gillibrand of New York have also reported that their offices received suspicious letters.

It is not yet clear just how many offices have been targeted by the mailings, though it appears that federal officials are approaching the incidents as being connected.

Two other sources familiar with the investigation into the suspicious letters, which contain an unknown white powder substance, said law enforcement has yet to trace the exact origins of the letters or the person or persons responsible for sending them.

According to one of the sources, “dozens and dozens” of letters have been sent out nationwide matching the same description.

“Everyone is at a loss at the scope of this, and no one knows where this is coming from,” another source told NBC News. “Most of the letters have been addressed to state offices but some to D.C.”

The FBI and the U.S. Postal Inspection Service are leading the investigation. Most if not all the letters sent to the senators were intercepted before they were delivered.

One of the sources said preliminary field testing continued through the night on the unknown substance and that so far, all the powder tested has been determined to not be hazardous.

Further testing will take place at law enforcement laboratories, the source added.

Senate and House staffers received notices on Thursday with warnings about the suspicious packages as officials began to investigate the letters.

As of Friday afternoon, state election offices in Alabama, Arizona, Connecticut, Delaware, Georgia, Idaho, Illinois, Kansas, Louisiana, Maryland, Massachusetts, Mississippi, Nebraska, New Hampshire, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, South Dakota, Tennessee, Washington, Wisconsin and Wyoming have also received packages.

The mailings come less than three weeks before Election Day and after threats against election workers have increased in recent years. Incidents of suspicious substances being mailed to election workers were reported both in 2023 and just weeks before the 2024 presidential election.