Investigators say they're puzzled by the significant amount of mercury found at a Las Vegas middle school, days after a few dribbles of the neurotoxin was first discovered and forced more than a thousand students into a quarantine that lasted overnight.

The U.S. Environmental Protection Agency said Monday that more than a quarter cup (59 milliliters) of the element has been cleaned up so far on campus at Walter Johnson Junior High School.

Clark County School District — the fifth largest school system in the country — says that it believes a student brought in the mercury but that there was no malicious intent behind it.

Mercury was first found at the school just before noon on Wednesday, prompting a screening for 1,200 students that lasted for 17 hours.