Roanoke County authorities are hoping an outside expert can tell them what's causing the mysterious "twitching" disease that has affected several students and a staffer at William Byrd High School.

A Richmond-based medical investigator from the U.S. Centers for Disease Control and Prevention will join a team trying to track down the cause of the mysterious symptoms, which also include headaches and arm spasms, the Roanoke Times reported.

Robert Parker, a spokesman for the Virginia Department of Health, said the CDC official represents an additional level of support for the investigation.

"We don't know where the finish line is," Parker told the Roanoke Times. "There may never be an explanation that we can point a finger to."

Epidemiologists are speaking with those affected and looking at their medical records, trying to determine what, if anything, they have in common. Did they all breathe the same air? Touch the same surfaces? Spend a significant amount of time together?

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About 30 students staged a walkout and protest earlier this week, and the school has said about 300 of the school's 1,200 students have been absent.

School officials say they do not plan to close the school.

Roanoke County school officials and health officials have found no environmental cause for the symptoms, which have affected fewer than 10 students. Still, high school officials can't specify specific symptoms to watch out for.

One mother said her child is experiencing bouts of sudden twitching and uncontrollable arm spasms, along with headaches and dizziness.

Health experts tested the school's air and water. They also looked for mold, but the school came up clean. Disturbed by the twitching that accompanies the illness, many students and parents want the school to close.

"They wave. It's convulsing. They can't stop it," said senior Layne Gulli of the symptoms. "You don't know how to avoid it. You don't know if you're next, or if your friend is next, or if it's an epidemic."

"There's rumors it was carbon dioxide from the photography room," said sophomore Joe Bradshaw. "We heard it was lead paint. Nobody knows what it is.

— The Associated Press contributed to this report