LE ROY, New York (REUTERS) - State health officials have added three more names to a growing list of students in this working-class town who are experiencing mysterious tics and twitching, while authorities on Saturday sought to assure parents the community's high school is safe.

Although the symptoms are typically associated with Tourette Syndrome, that has been ruled out in all but one case, causing fear and confusion among many residents of Le Roy, N.Y., about 50 miles east of Buffalo.

The building is safe for the community, District Superintendent Kim Cox told several hundred residents gathered in the auditorium of Le Roy Junior-Senior High School on Saturday.

The Le Roy Central School District scrambled to conduct environmental testing for air quality and mold when an initial 12 students developed tics and impulsive verbal outbursts last fall. But state health investigators ruled out environmental factors, latent side-effects from drugs or vaccines like Gardasil, trauma or genetic factors.

Instead, doctors say conversion disorder - once called mass hysteria - is to blame among an expanding list of patients. Three more unconfirmed cases have been added to the original list of students exhibiting the symptoms, and others are being examined.

Air quality and mold surveys at the school have all come back negative, according to district officials and representatives of Leader Professional Services Inc., a company hired to conduct environmental testing at the school after the symptoms first surfaced.

Senior Industrial Hygienist Mary Ellen Holvey on Saturday said air and water tests turned up nothing, and recommended follow-up testing of air inside the school.

She said that would help determine whether a soil review will be conducted - a test demanded by those residents who believe environmental factors are to blame.

One parent, Melissa Cianci, said her daughter no longer wants to attend school in light of the outbreak. She said students should be moved to another location as the investigation continues.

She doesn't know if it's safe, Cianci said, adding her daughter had perfect attendance prior to the incidents. I'm done listening to you, she yelled at the panel before storming out, later criticizing the district for being less than candid early in the investigation and demanding that soil tests be conducted of school grounds.

Though there is no evidence of environmental contamination, for some residents environmental concerns were heightened by the district's recent disclosure of six natural gas wells on school property, as well as possible contamination from the nearby site of a 1970 train derailment and chemical spill.

Regarding the wells on school land, William Albert, of the district's law firm Harris Beach, said, It's not unusual. We're out in the country.

Several representatives of renowned environmentalist Erin Brockovich were barred recently from collecting soil samples near the school by local police.

State health officials note that all of the patients have had significant stress factors, which can worsen the condition. Three of them had pre-existing medical conditions, including one confirmed case of Tourette's Syndrome. Just one of the patients in male.

Congresswoman Kathy Hochul, who represents the district, sent a letter to the environmental Protection Agency on Monday calling for a review of the Superfund site, which the EPA said is regularly monitored, including testing scheduled later this month.

(Editing by Paul Thomasch)