A recently crowned US beauty pageant winner faces charges of bomb possession.
Kendra McKenzie Gill was arrested last weekend with three accomplices for what one described as a prank.
All four face the same set of four felony charges, prosecutor Blake Nakamura said on Friday.
The 18-year-olds were arrested last Saturday after driving around neighbourhoods and allegedly tossing plastic bottles filled with caustic chemicals at people they knew.
Nobody was injured.
"We don't really understand a clear reason for their behaviour," Nakamura said.
"The reason we charged them is obviously, what they possessed was indeed explosives, and we're alleging they were throwing them near homes and at people, and therefore, had the potential to cause a great deal of harm."
Felony bomb possession is punishable by 1 to 15 years in prison.
Gill was crowned Miss Riverton, topping a slate of nine beauty contestants earlier in the northern summer in the Salt Lake City suburb.
She showed off years of piano training with a Scott Joplin number and took home a $US2000 ($A2210) scholarship.
Others charged on Friday were John Patrick Reagh, Shanna Marie Smith and Bryce Christopher Stone.
All four are due to make their first court appearance together on August 26 in Salt Lake City.
Each has been released on $US50,000 bond.
Nakamura said some or all of the teens had lawyers, but he didn't know their names.
All four acknowledged they were tossing bombs powered by a toilet bowl cleaner reacting with aluminium foil on footpaths, front yards or in streets "to scare some of their other friends," Unified Fire Authority investigator Steve Bowen said in a court affidavit filed on Friday.
Stone reportedly told authorities it was a prank.
"They were throwing them at both property and people," Unified Fire Authority Captain Clint Mecham told KUTV-TV earlier this week.
"This goes well beyond a teenage prank."
