LAS VEGAS (AP) — 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 in western Las Vegas. “It’s a pretty significant amount. It’s definitely more than a thermometer-worth,” said EPA spokesman Rusty Harris-Bishop. “I don’t know how you would get a hold of it (in that quantity).” Clark County School District — the fifth largest school system in the country — maintained Monday that it believes a student brought in the mercury but that there was no malicious intent behind it. An investigation into how it got to the school is being led by school police. Classes at the school were cancelled Monday for a third day.