The 16-year-old was attending a large party near a campground last Friday night.
Lindsey Rodni-Nieman got a text late Friday night from her teenage daughter, saying she was planning to leave a party in about 45 minutes and would be coming "straight home."
"I told her to be safe and that I loved her. And she said, 'OK, mom, I love you, too,'" Rodni-Nieman recalled in an interview with ABC News on Monday. "She never came home."
Kiely Rodni, 16, was last seen early Saturday around 12:30 a.m. local time near the Prosser Family Campground in the small, Northern California town of Truckee, some 20 miles north of Lake Tahoe. She was at a party with upwards of 300 people when she vanished along with her vehicle, a silver 2013 Honda CRV with California license plates, according to the Placer County Sheriff's Office.
Her phone has been out of service since then, according to the Placer County Sheriff's Office, which is leading the ongoing investigation and search.
"Her cellphone went dead and became virtually untraceable shortly after," Angela Musallam, public information officer for the Placer County Sheriff's Office, told ABC News during an interview Monday.
When Rodni's mother awoke that morning to find her daughter still not home and her car missing from their driveway, panic began to set in.
"I called her and sent her texts, too, and she didn't answer," Rodni-Nieman said. "That's when I knew something was wrong."
She said she doesn't believe her daughter ran away because all her clothes and belongings are still at home, and "it was already so out of character" for her to not call or text back.
Rodni's friend, Sami Smith, said she was the last person to speak with her at the party early Saturday.
"She was having a fun time at a party, just being a teenager," Smith told ABC News during an interview Monday. "Everything she drank, I drank out of, and there was nothing that seemed off about her."
"I never expected this to happen ever," she added. "Nobody in this town did."
With no trace of Rodni or her car, detectives are not ruling out a possible abduction, Musallam had said. Though, "right now we don't have any evidence that supports an abduction," Placer County Sgt. Scott Alford told reporters during a briefing Tuesday.
"We're considering everything," he said. "This is a missing person's case, this is a search-and-rescue effort."
The Placer County Sheriff's Office has urged anyone who saw her that night to come forward, and so far investigators have received over 200 tips, Alford said Tuesday.
Other local, state and federal agencies, including the Truckee Police Department, the Nevada County Sheriff's Office, the California Highway Patrol and the FBI, are assisting the Placer County Sheriff's Office in the investigation, according to Musallam.
On Tuesday, helicopters from the California Highway Patrol and the Placer County Sheriff's Office continued to conduct aerial searches for the teen and her car.
Authorities, as well as Rodni's family, are urging anyone who attended the party to cooperate with the investigation. In particular, investigators are asking for any photographs or videos from that night to help them piece together a timeline. A $50,000 reward is being offered for information that leads to Rodni's safe return.
"We're just begging, begging for you to please come forward and share your story," Rodni-Nieman said.
Anyone with information about Rodni or her whereabouts can call the Placer County Sheriff's Office's tip line at 530-581-6320 and select option seven. Callers can remain anonymous.
ABC News' Meredith Deliso contributed to this report.