Alyssa Byrne, 19, was with friends attending a music festival in South Lake Tahoe before she vanished. She was last seen shortly after midnight on New Year's Eve at the Horizon Casino Resort in Stateline, Nev.
Sheriff's deputies are now trying to trace newly discovered activity made with Byrne's cellphone after she disappeared.
Douglas County investigators confirmed Thursday that either a call or text was placed from her phone after midnight. Investigators had originally believed that the last activity on her cellphone was made before midnight.
But, investigators cautioned, the activity does not mean that the phone was in Byrne's possession.
Byrne was attending the Snow Globe Music Festival on New Year's Eve, but she left the party and took a shuttle back to the hotel. Investigators said they have a credible eyewitness who talked to her in the casino. No one has seen her since. Byrne's friends looked for her but went home with her things when it was time to check out of the hotel.
A Twitter user by the name of "@alyssa_byrne1" posted on New Year's Eve: "After tonight, you'll never be a part of my life." Another tweet by the same user said, "Just lost all my trust in the person I trusted most."
"We've seen those and there's still a debate as to the actual author of those tweets," Douglas County Undersheriff Paul Howell said.
Teams of investigators are looking through hours of surveillance tape from the casino -- a job that could take days and may not produce any leads.
In Petaluma, friends and family from far and wide have gathered under the same roof to support the Byrne family.
"They've brought food, they've brought drinks, they've brought hope," said Kim Byrne, Alyssa's mother.
Friends and family are trying to stay positive. Alyssa Byrne has always been a good girl, not a person to drop out of contact, worry her parents or leave her valued stuff behind.
"She would not take off without her laptop. She would not take off without at least a couple of toiletry items. She would not take off without her dog," Kim Byrne said. Flyers have been distributed and a Facebook page that now has thousands of hits has also been created.
To family members, it's all a bit surreal. "You think it only happens to other people and you can't imagine it's happening to you or someone you know and love," said Sam Miller, Alyssa's sister.
Kim Byrne said the family won't stop looking until Alyssa is home. "We are in it to win it," she said.
Sheriff's deputies told ABC7 News that finding Byrne is their top priority. Investigators say there no signs of foul play, but that it is highly unusual for Alyssa to just disappear without contacting anyone. The search will resume Friday morning. Fundraisers will be held at Beyond The Glory in Petaluma tomorrow night and at Maverick's in Santa Rosa.