"It's important to hold onto hope," Rodni's mother, Lindsey Rodni-Nieman, said.
Authorities in Northern California have announced a new phase in the ongoing search for a teenager who disappeared after a party 11 days ago.
"We are moving into a more limited but continuous search-and-rescue effort," Capt. Sam Brown of the Nevada County Sheriff's Office said during a press briefing on Monday. "We are going to have to switch modes and kind of focus on the investigative end and try to figure out where do we go from there."
Kiely Rodni, 16, was last seen on Aug. 6 around 12:30 a.m. local time near the Prosser Family Campground in the small 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 and a sticker of a ram below the rear wiper blade, according to the Placer County Sheriff's Office, which is leading the search and investigation.
Rodni's cellphone has been out of service since then.
"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 that aired Aug. 9 on "Good Morning America."
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 press briefing on Aug. 9.
"We're considering everything," Alford said. "This is a missing person's case, this is a search-and-rescue effort."
Dozens of law enforcement personnel have been involved in the search, including foot patrol, aircraft, canine and dive teams. 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.
Rodni has also been added to the FBI's missing persons database. Among the more than 1,200 tips the FBI said it has received and combed through, investigators have pursued several leads, including digging up a burial site near the Prosser Family Campground -- only to find the remains of a dog.
"It's important to hold onto hope," Rodni's mother, Lindsey Rodni-Nieman, told ABC News during an interview Tuesday on "GMA." She then added: "It's OK to feel sad and frustrated, it's OK to feel this anguish."
Rodni-Nieman told ABC News the last text message she received from her daughter said she was planning to leave the party in about 45 minutes and would be coming "straight home." That was about an hour before Rodni's cellphone last pinged near a lake.
Authorities, as well as Rodni's family, are urging anyone who saw her the night she vanished to come forward as well as 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.
Last week, the Placer County Sheriff's Office said its detectives have located surveillance footage from a local business in Truckee where Rodni was spotted on Aug. 5 at 6:08 p.m. local time, prior to her disappearance. She was last seen wearing a black spaghetti-strap bodysuit, green Dickies pants with a black belt and black Vans shoes. She also may have a dark gray Lana Del Ray hoodie with the lyrics: "You don't want to be forgotten. You just want to disappear."
However, on Sunday, the Placer County Sheriff's Office said its detectives "have developed information" that Rodni was seen in another video, taken that night at the party, wearing a white sweatshirt with the pink writing "odd future." It remains unclear whether she was wearing that sweatshirt when she vanished.
"I recognize both sweatshirts," Rodni's mother told ABC News. "The darker one belongs to her best friend. The lighter one is something that I've seen her wear time and time again."
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 and Veronica Miracle contributed to this report.