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Alyssa Byrne of Petaluma missing in South Lake Tahoe. (FACEBOOK PHOTO)

A Petaluma teen who disappeared New Year's Eve after attending a South Lake Tahoe music festival remained missing Thursday amid mounting concerns for her safety and a sense that something wasn't quite right with her when she vanished.

Friends who saw Alyssa Byrne, 19, shortly before she went missing said she wasn't acting like herself when she unexpectedly left a New Year's Eve concert an hour before midnight and, later, passed on a chance to visit with old high school friends at her hotel.

Douglas County, Nev., sheriff's Sgt. Pat Brooks said Thursday afternoon that investigators found Byrne's cell phone was last used at three minutes past midnight. Investigators are also reviewing surveillance footage from all casinos in the area, a process that would take at least a full day, he said.

The last friend to see her, Micah Alex of Petaluma, said it was his impression that Byrne was under the influence of alcohol or drugs when they embraced and greeted in a ground-floor passage near the lobby of the Horizon Casino Resort before she went missing.

"She just seemed like something was wrong with her," Alex said Thursday, "like she wasn't all there, you know?"

Last Saturday, Byrne and friend Jay Donnellan, a high school classmate, and two other friends from Napa drove to South Lake Tahoe for the outdoor SnowGlobe Music Festival, a three-day event on the Lake Tahoe Community College Campus that began at 3:30 p.m. Saturday. The festival draws upwards of 40,000 people.

The four shared at a room at the Horizon Casino Resort in Stateline, Nev., 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.

"We're all really scared," Alex said Thursday morning.

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.

Brooks said people go missing at Tahoe often, but frigid winter temperatures add to the urgency to find Byrne.

He said investigators are offering a reward of up to $1,000 in hopes someone will come forward with information.

"There's a lot of snow up at Lake Tahoe. The weather is cold," Brooks said. "We need to know if Alyssa's fine or not. We need to find her."

Byrne, a 2011 graduate of Petaluma's Casa Grande High School, lives with her parents, Kevin Byrne and Kimberly Miller-Byrne. She works as a hostess at the Cattlemens restaurant in Petaluma and attends Santa Rosa Junior College. Her plan is to become a paramedic-firefighter, her mother said.

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

He said she called his cell phone twice on New Year's Eve. He missed the first call but around 11:30 p.m she got through to him, saying she was on a shuttle bus back to the Horizon.

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

When Byrne was not in the hotel room they shared, Donnellan called her cellphone, but it went unanswered -- as it would each time he called that night, until it began going straight to voicemail the next day.

Friend Micah Alex said he last 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 he told her were nearby.

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.

Donnellan said that he did not believe that she had consumed anything that would have impaired her behavior that day, other than beer.

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 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 were not 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 going. I don't know."

By morning, conversations between friends and Byrne's father had intensified.

Donnellan and the others delayed their departure from the Horizon -- until security was about to kick them out -- 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, Nev., in which the Horizon is located, though not until after 3 p.m. Thursday, because of the New Year's holiday, authorities said.

Kevin Byrne said he has been impressed with the work of both departments and has a cadre of friends and relatives working with him to distribute fliers of his daughter. There are two site on Facebook dedicated to her disappearance.

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.

Staff writer Kristin J. Bender and Santa Rosa Press Democrat contributed to this report.