Sen. Ted Cruz’s campaign office in Houston received a “powdery substance” in the mail on Tuesday, prompting a fire department investigation, the Texas Republican’s campaign confirmed Tuesday. 

Cruz spokeswoman Emily Miller confirmed the incident and wrote in an email that a piece of mail with a powdery substance was addressed to the campaign headquarters. The incident was first reported by the Weekly Standard.

“Authorities were alerted and are on the scene,” Miller wrote. “The building is still secured for entrance and exits by fire and police. We don’t yet know what was the substance.

The Houston Fire Department tweeted that two people were taken to the hospital after being exposed to the substance. Miller said no campaign staffers were taken to the hospital, and she does not know who they are.

Cruz is in a competitive race against Democratic Rep. Beto O’Rourke.