The four shared a room at the Horizon Casino Resort in Stateline, and spent most of the ensuing days together, shuttling between their hotel and the festival, Donnellan said.

Monday night, New Year's Eve, they were outdoors at the festival around 11 p.m. when Byrne got slightly ahead of her friends as they made their way through the crowd and then, suddenly, disappeared, Donnellan said.

Byrne had left a bit earlier than her friends a night earlier, after falling in a puddle and getting wet enough that the outdoor temperature was unbearable, he said.

But it was odd for her to leave without saying anything or even alerting them she planned to go, Donnellan said.

But Byrne called his cell phone twice. He missed the first call but around 11:30 p.m she through to him, saying she was on a shuttle bus back to the Horizon, he said.

About 20 minutes later, her friends tried to leave the event but were held up by midnight fireworks and crowds in the street that delayed their return to the hotel until about 12:30 p.m.

When Byrne wasn't in the hotel room they shared, Donnellan called her cell phone, but it went unanswered - as it would each of the 10 or 15 times he called that night, until it began going straight to voicemail the next day.

In the meantime, Alex - who was among several groups of young people from Petaluma attending the festival - crossed paths with Byrne in a passageway between the Horizon lobby and a bar. She had put her hair in a pony tail and was wearing a black sweatshirt, not the heavy, white winter jacket she had had on earlier at the festival.

They hugged and exchanged greetings but she didn't seem herself, Alex said, and showed no interest in visiting with his girlfriend, a close high school friend, or several other classmates with them.

Donnellan said he was not aware of her having consumed anything besides beer, but Alex said she appeared to be under the influence of drugs or alcohol, and clearly would have been too impaired to drive.

Usually very social, she "was just kind of in her own little world," Alex said. "That was the last time we saw her."

That was sometime between 12:30 and 1 a.m. New Year's Day.

Alex was among those with whom Donnellan talked that night as the hours went by and he and his friends were unable to locate Byrne.

Donnellan and the others in their traveling party went out until about 5:30 a.m., in part enjoying the evening and in part looking for Byrne. Two other friends stayed back at the hotel room in case she returned, he said.

Donnellan said he figured, at the time, Byrne might have gone off with a girlfriend, though she'd made no mention of meeting anyone new or running into someone the others weren't aware of.

He said his friend is strong and smart enough to take care of herself, and would never "wander off with some random guy."

"It was almost like she was ignoring our phone calls for some reason. I mean, I don't know — like she was meeting up with somebody (and) she didn't want us to know where she was doing. I don't know."

By morning, conversations between friends and Byrne's father had intensified as they struggled for answers.

Donnellan and the others delayed their departure from the Horizon — until security was about to kick them out, he said — as they checked with other hotels, hospitals, the jail, the sheriff's department and any place the could think of, Donnellan said.

They filed a missing person report with the South Lake Tahoe Police Department. The case was then referred to the sheriff's department in neighboring Douglas County, Nevada, in which the Horizon is located, though not until after 3 p.m. Thursday, because of the New Year's holiday, authorities said.

But Kevin Byrne said he's been impressed with the work of both departments and has a cadre of friends and relatives working with him to distribute fliers about his daughter's disappearance. Thursday, he was fielding multiple calls from media outlets around the nation interested in the story.

He was at the sheriff's office until midnight Wednesday as investigators combed through Horizon surveillance tape in an unsuccessful search for images of Byrne, and again on Thursday morning, while a search and rescue effort was organized on his daughter's behalf.

Byrne said he hoped search and rescue crews would not find her Thursday.

"A search and rescue operation today would be a search and recovery, as far as I'm concerned," he said.

Byrne is described as 5-feet-3-inches tall, weighing about 125 pounds with fair skin, dyed black hair and blue eyes. She was last seen wearing a black sweatshirt, black snow boots, and black yoga-type pants with her hair pulled back.

Anyone with information about her is asked to contact Douglas County dispatch at (775) 782-5126, or Investigator Dennis Slater, at (775) 586-7255. An anonymous tip line is (775)782-CRIME.