LAS VEGAS — Hundreds of anxious parents staked out a local middle school after mercury was found and federal officials kept more than 1,000 students for up to 17 hours to screen them for exposure to the neurotoxin.

Authorities were investigating how mercury got into Walter Johnson Junior High School on Wednesday, forcing 1,300 students, teachers and first responders to undergo testing by the U.S. Environmental Protection Agency, according to Clark County School District police Capt. Ken Young.

No illnesses were reported, although the fire department took two adults to a hospital after they suffered unrelated medical episodes while waiting outside the school.

The last people were released at 5 a.m. Thursday, and school was canceled. The mercury was discovered at noon Wednesday, and automated messages alerted parents to the hazmat situation.

High levels of exposure can cause mercury poisoning with symptoms including muscle weakness and speech, hearing and walking impairment, the EPA’s website says.

The fire department helped EPA workers conduct the screenings, which took about five minutes per person, though some had to be cleaned and checked multiple times.

He said it was the largest decontamination response the department has ever handled.

One by one, the 1,200 students were swiped with a wand that checked for mercury residue. They also threw their labeled shoes and backpacks into a garbage bag to be examined separately.

Contaminated students had to be cleaned off, ranging from wipes for their hands, to dipping their feet into a tub of chemical soap, washing their hair and changing into a set of school-provided physical education uniforms.

Everyone was eventually cleared of mercury residue. The level of exposure among the contaminated students was minimal because no one is believed to have ingested it.