After accidents with anthrax and a deadly flu strain in recent weeks, the Centers for Disease Control and Prevention temporarily halted the shipment of samples from its high-security laboratories and said it will review safety procedures with external advisors.

CDC Director Tom Frieden on Friday said a lab that works regularly with flu viruses at the agency had accidentally cross-contaminated a low-pathogenic H9N2 virus sample with a strain of H5N1 flu, one of the most deadly viruses known. The sample was then shipped to a...