ANNAPOLIS, Md. (AP) — PharmAthene shares plummeted in morning trading Friday after the company said health regulators halted its study of an experimental anthrax vaccine.

The company said the Food and Drug Administration placed a clinical hold on the company's trial of SparVax rPA trial. The agency did not give a reason, but plans to send PharmAthene a letter explaining the action in the next 30 days.

The clinical study, which was expected to begin by the end of this year, has not enrolled any patients yet.

Company shares fell 20 cents, or 13.5 percent, to $1.26 in morning trading. They are near their 52-week low of $1.07 set in mid-December.