They never told us that. It was suggested as a possibility very early before more facts were obtained.
If you required everyone in government to refrain from speaking until they knew something definitive, you'd probably accuse them of engaging in coverup or denial.
http://www.cnn.com/2001/HEALTH/10/05/florida.anthrax/
http://www.cdc.gov/ncidod/EID/vol8no10/pdf/02-0389.pdf