U.S. federal health officials confirm that contaminated vials linked to cases of meningitis do indeed contain the fungus responsible for causing the outbreak.

Early tests on a batch of steroid led to a strong suspicion of a link. The latest tests announced Thursday by the Food and Drug Administration confirmed it.

The meningitis outbreak has killed 20 people so far. There are at least 257 confirmed cases in 15 states. But officials believe as many as 14,000 people in 23 states were injected with the tainted medicine for back or joint pain.

It is unclear how many people will get sick. Doctors say it could take several months for meningitis symptoms to appear.

The now-closed New England Compounding Center in Massachusetts made the steroid. All the suspect medicine has been recalled. Federal authorities are investigating the center for allegedly breaking the law by making mass doses of a medicine. The pharmacy was only licensed to make small doses of made-to-order prescriptions.

Meningitis is a disease infecting membranes that protect the brain and spinal cord.