LAS VEGAS (AP) — Hundreds of anxious parents staked out a Las Vegas middle school after the discovery of a small amount of mercury led federal officials to keep more than a thousand students for up to 17 hours to screen them for exposure to the neurotoxin.

Authorities were investigating if a student brought the substance to 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.

It’s too early to say what sort of charges or discipline could be imposed, he said. No mercury-related illnesses have been reported.

Some parents criticized the lack of information they received and the intensity of the response, but officials said it was important to be sure no one went home contaminated.

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

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

They waited outside for word on their children, who slowly trickled out through the evening and into the next morning.

Lori Barga told the Las Vegas Review-Journal that she was worried about her 11-year-old daughter.

“They have been in there since 8 o’clock this morning. Class was supposed to finish at 3:20 p.m. and now it’s past midnight,” Barga said early Thursday. “I’m worried that they will be tired and hungry and stressed.”

Students were quarantined in their classrooms until they were screened but were allowed to use the restroom. They received water, juice and food. First responders also coordinated with parents to deliver medicine to children as needed.

“They were laughing. They were talking. They were really, really upbeat,” fire department spokesman Tim Szymanski said of students’ morale. “The kids didn’t complain or seem out of it. I was there really, really late and I was really amazed at their attitude.”

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. By 11 p.m. Wednesday, a fourth mercury-sniffing machine was flown in from California to speed up the process, Szymanski said.

It was the largest decontamination response the department has ever handled, he said.

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, said Young, the school district police captain.

A teacher initially reported the substance after seeing a group of boys and girls playing with it during a school assembly in the gym. The fire department confirmed the silvery liquid was mercury.

The EPA said it’s still evaluating how to decontaminate the school, which can take up to a week. The agency will check the entire property and screen all of the students’ personal items before they are returned. A town hall meeting is set for Thursday evening to answer questions.

Young said upset parents have complained about not getting enough information and some even wanted to refuse the EPA screening to get their children back.

Some of them also criticized the response as being more burdensome than necessary because the small amount that spilled at the school roughly equaled what can be found in an old thermometer.

“How would the headline read if we sent kids home contaminated and we didn’t check?” Young said.

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