TUCSON, Ariz. — White powder delivered in an swastika-covered envelope to the Tucson, Ariz., office of Rep. Raul Grijalva is a toxic substance, the Democratic congressman said the FBI told him Thursday afternoon.

It was not confirmed exactly what substance was mailed to Grijalva's offices, located at the 800 block of East 22nd Street, NBC affiliate KVOA reported.

A staff member called Tucson Police earlier Thursday after finding an envelope containing the suspicious white powder. The offices were locked down by Tucson Police, and the FBI arrived to assist in the investigation, KVOA said.

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An envelope arrived with swastikas drawn on the outside and a white powdery substance on the inside, Adam Sarvana, Grijalva’s spokesman, told the Arizona Daily Star newspaper.

It’s not the first time his office has been the target of threats, the Daily Star said.

Grijalva received death threats in April, shortly after SB1070, the anti-illegal-immigrant law, was signed. He called for a boycott in response. He closed his Tucson and Yuma offices as a result of the threats, the Daily Star said.

In July, after he called off the boycott, staffers found a bullet and a shattered window inside his Yuma office, the Daily Star said.

Grijalva appeared on a national news talk show this week complaining about racist tactics being used during his campaign for re-election.

Republican opponent Ruth McClung told the Daily Star she condemns the latest news and said violence or threats of violence against politicians are wrong.