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A 7-year-old girl was killed and her 3-year-old sister is still missing after flash flooding was reported in a canyon near a state park in Utah on Monday when a strong thunderstorm rolled through the area, triggering a massive search for the missing child after a piece of her clothing was found miles away.

The Emery County Sheriff’s Office said in a news release on Facebook that a multiagency search is underway in the Little Wildhorse Canyon near Goblin Valley State Park after the isolated thunderstorm caused flash flooding in the area.

"It is with great sadness that ECSO confirms that the deceased victim of yesterday's flash flood in Little Wildhorse Canyon is a 7-year-old Utah girl," the sheriff's office said Tuesday morning.

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The search of the area continues on Tuesday for the girl's missing 3-year-old sister.

"A piece of clothing believed to belong to the missing 3-year-old was found last night approximately 7 miles down a wash from the staging area," the sheriff's office said.

At least 21 people made it safely out of the Little Wildhorse Canyon after an isolated thunderstorm caused flash flooding in slot canyons on Monday. A 7-year-old girl died and her 3-year-old sister is missing, according to police.

At least 21 people made it safely out of the Little Wildhorse Canyon after an isolated thunderstorm caused flash flooding in slot canyons on Monday. A 7-year-old girl died and her 3-year-old sister is missing, according to police. (Emery County Sheriff’s Office)

Crews searched until late Monday night and resumed on Tuesday morning.

A total of 21 people made it out of the canyon. Officials initially said that vehicles parked at trailheads lead searchers to initially believe that there were still more hikers, but all have since been accounted for.

"ECSO expresses our deepest sympathy to the family that has suffered this tragic loss," the sheriff's office said. "We also express gratitude to all who are assisting in this search."

The search of the area is being conducted by 67 personnel from multiple agencies, including search and Rescue teams from Emery County, Grand County, Sevier County, and Wayne County, along with personnel from the Bureau of Land Management, Utah State Park, Utah Division of Wildlife Resources, and Utah Highway Patrol.

Six ambulances from Emery, Wayne, and Sevier counties are standing by on scene at the canyon located about 227 miles south of Salt Lake City.

According to the sheriff's office, two helicopters from Classic Air in Moab and one helicopter from the DPS Aero Bureau were also assisting with the search.

Officials did not say how many people were possibly injured when the flash flood occurred.

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The National Weather Service (NWS) office in Salt Lake City tweeted Monday afternoon to announce the thunderstorm approaching the Goblin Valley State Park area and Little Wildhorse Slot Canyon had "continued to intensify."

"Expect nickel sized hail and strong winds with this storm," forecasters said.

That storm then crossed directly over the state park and the San Rafael Swell, triggering the flash flooding in the slot canyons.

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The NWS forecast office said Monday night that showers and thunderstorms over Utah and southwest Wyoming also brought areas of strong winds to the region, with peak gusts of up to 61 mph reported.