Brooklyn went missing from her home around 8 p.m.; she was last seen wearing black pajama pants and a gray T-shirt, The Salt Lake Tribune reported.
"This is our big push today and we have about 300 people who showed up to help us," Unified Police Department Lt. Justin Hoyal said Thursday. "The community's response has been awesome. People are really showing great support for our search."
Hoyal said Brooklyn had a history of running away from home, but earlier incidents were "not similar to those now" and that "the places she would or could have gone, she hasn't gone there."
"At this point, we can't rule out that she ran away or that something suspicious happened," Hoyal said. "There were no signs of a forced entry in her home. Our big concern is that she's out there, ill-prepared with her clothing."
Overnight temperatures in the area have been in the teens, the news report said.