Packages containing white powder were delivered to the offices of Sen. Mark Begich (D), Sen. Lisa Murkowski (R) and Rep. Don Young (R) in Alaska on Monday.

The Fairbanks Daily News-Miner reported the incidents. The packages were opened in Young’s Anchorage office and Begich’s Fairbanks office before the powder was discovered, but Murkowski’s staff did not open the package sent to her in Fairbanks.

The FBI determined that the powder sent to Young was a concrete mixture and not hazardous, the Republican’s office said in a statement late Monday. Staffers will return to work at the location Tuesday.

The statement said it was unknown why the package was sent to the Congressman.

Begich spokeswoman Julie Hasquet told the News-Miner that the package had an Arizona return address. The aide who opened it was following standard safety practices, and there was no sign of exposure, Hasquet said. However, the aide was quarantined while tests were conducted on the powder.

Begich’s Anchorage office was also targeted, Hasquet said without elaborating.

“It doesn’t appear to be anything more than a hoax, but we don’t have that confirmed,” she told Reuters.

This year marks the 10th anniversary of the deadly 2001 anthrax attacks on Senate offices and other targets.