A hazardous material emergency response team was sent to the campaign offices of Sen. Ted Cruz Tuesday after staffers opened an envelope “containing a white powdery substance.”

According to the Weekly Standard, fire trucks and first responders arrived minutes after Cruz campaign workers reported the incident, with eyewitnesses saying at least one hazmat truck was seen on the premises.

“Two people were taken to the hospital after apparently being exposed to a white powdery substance in an office building at 3200 SW Fwy. The 9th floor of the Phoenix Tower has been evacuated as HFD HazMat is responding to the scene working to determine the nature of the substance,” confirmed the Houston Fire Department.

Two people were taken to the hospital after apparently being exposed to a white powdery substance in an office building at 3200 SW Fwy. The 9th floor of the Phoenix Tower has been evacuated as HFD HazMat is responding to the scene working to determine the nature of the substance. — Houston Fire Dept (@HoustonFire) October 2, 2018

It remains unclear of the substance tested positive for ricin or other toxic materials.

This is a developing story. Check back for updates.