State health officials continue to quietly investigate what may have caused one to two dozen students to exhibit unexplained vocal tics or repetitive hiccups at Essex Agricultural and Technical School in Danvers and, to a lesser extent, North Shore Technical High in Middleton, this past school year.

The state is reaching out to more than 2,600 doctors in the region while investigating environmental factors that may have caused the symptoms, including air testing and visual observations inside school buildings. So far, according to a May 10 status report from the state Department of Public Health, air tests did not turn up anything that would contribute to "significant neurological effects."

Danvers Health Director Peter Mirandi, in an update to the Board of Health on May 30, said the state is casting a wide net to gather information from doctors and their patients.

"They will be looking for correlations between the symptoms and the students," Mirandi said. "The effort from the Department of Public Health is very thoughtful, deliberate, and I wouldn't mind recognizing them for stepping up when we needed assistance."

A spokesman for the state Department of Public Health had no update on the investigation.

"It's a very extensive process, and there is no report that I have," said Ann Roach, media relations manager for the state Department of Public Health.

In February, state public health officials met with parents of affected students, school officials and Danvers and Middleton health officials to learn more about their concerns.

In March, the Department of Public Health and the Board of Registration of Medicine sent letters to more than 2,600 "attending physicians, encompassing more than a dozen medical specialties" looking to see if any had treated patients with vocal tics or chronic hiccups from the two schools. The doctors were asked to discuss the state investigation with parents to see if they would consent to allow the state to obtain the patients' medical records.

"To date, MDPH has received signed medical records forms for nine students who have been reported as having these symptoms," according to the May 10 status report.

The state has also conducted a number of indoor air quality tests and made visual observations in school buildings.

"Based on the observation and air measurements taken during the building investigations conducted at Essex Agricultural and Technical High School to date, no environmental factors that would be most likely attributed to significant neurological effects have been detected," the update states.

The state is also looking into environmental information for the East Street field in Middleton "where parents who attended the February meeting reported that many of the students experiencing vocal tics and chronic hiccups have played sports."

In January, some health experts who had not seen any of the local students theorized the incidents appeared to be a case of conversion disorder and mass psychogenic illness, also known as mass hysteria. That was the explanation given for a widely publicized case involving high school girls suffering tics in Le Roy, N.Y. It's a diagnosis that came under intense scrutiny.

Parents from around the country supported the theory that pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections, or PANDAS, may be to blame. This condition involves rare cases when strep infections cause autoimmune reactions in the brain.

The Essex Aggie case has caught the attention of New Zealand author Robert Bartholomew, a native New Yorker who has a doctorate in medical sociology and who teaches at the Botany Downs Secondary College in South Auckland. The researcher has written extensively on the subject of mass hysteria in schools.

"This is a highly unusual case. After all, it's not every day that more than a dozen high school students come down with vocal tics and hiccuping," he said in an email. "It would be almost impossible to fake the symptoms ... being described," said Bartholomew.

He has been trying, with no luck, to get answers from the state Department of Public Health to learn from these incidents so they could be prevented from recurring.

"Transparency is the best policy in these cases," Bartholomew said.