The letter addressed to Rep. Greg Stanton was intercepted before reaching his office. Letters sent to Reps. Yassamin Ansari and Adelita Grijalva went to outdated office addresses, according to information provided to the lawmakers by the FBI’s Phoenix Field Office and the Arizona Department of Homeland Security.

Ansari said all three letters contained white powder. It is not yet known whether the material was toxic, she said.

“Threats of political violence have gotten so extreme,” Ansari said in a radio interview Monday, adding that she would be hard-pressed to find a colleague who had not received some form of death threat. She said congressional staff are receiving training in response to threats.

The FBI confirmed that letters had been sent to congressional and secretary of state offices around the country, though only some contained unknown substances. The bureau said it is coordinating with law enforcement partners to address each incident and identify whoever sent the letters.

The latest reports follow a separate incident Thursday at the Arizona Executive Tower, where the Secretary of State’s Office was evacuated after staff opened an envelope containing white powder. Officials later determined that powder was not toxic and cleared the office and a mail-sorting facility.

Arizona Secretary of State Adrian Fontes has said that false claims about elections can contribute to threats against election officials. Authorities have not publicly linked the congressional letters to the earlier incident or identified a motive. The lawmakers said they would continue their work and not be intimidated.