Mikulski, the top-ranking Democrat on the Senate Appropriations Committee, peppered Obama administration officials with questions in a May 28 letter released to the public Monday.
"I am aware that both the DoD and CDC are investigating this incident, but I have serious concerns about how such a potentially catastrophic mistake could have been made," she wrote to Secretary of Defense Ash Carter and CDC director Thomas Frieden.
"Given the gravity of this matter, and the possible public health implications, I'm sure I share your concerns in wanting to isolate the effects of this incident," she added.
The Army Edgewood Chemical Biological Center at Aberdeen Proving Ground was one of the labs that received the samples. The Edgewood center, which describes itself as the nation's principal research and development resource for non-medical chemical and biological defense, transferred some samples it received to other labs in the United States.
None of the people officials said were being treated last week for possible exposure to the deadly bacterium are based in Maryland.
Chemical weapons specialists from the Edgewood Chemical Biological Center last year destroyed Syria's stocks of the World War I blister agent sulfur mustard and the sarin precursor DF.
Anthrax is an acute bacterial disease that can be fatal if not treated. It is not contagious but can be inhaled, ingested or transmitted through direct contact on the skin. Someone who is infected might not show symptoms for weeks.