LOS ANGELES (CBSLA.com) — The mystery surrounding missing hospital volunteer Caroline Karimi  was abruptly solved Wednesday evening when the missing teen’s mother told KCAL9 her daughter simply went on a road trip.

Karimi’s mom said her daughter called from Cheryenne, Wyoming.

It is believed Karimi went on a road trip with two friends and former classmates, Vivian and Valerie Lopez.

Karimi’s mother arrived home from Boston where the missing woman and her father were expected to join her today on vacation.

The missing 18-year-old woman had last been seen Monday on surveillance video leaving a store near her home where she purchased lunch.

Karimi was seen driving off in her green 2007 Prius (California license plate  5YVV475). No one was seen following her on the video.

She had just texted her mother saying she was about to make the 40-minute drive to work, a volunteer job at San Gabriel Valley Medical Center. She never arrived there.

The woman’s frantic father, Dr. Ardeshir Karimi, said today that his daughter had a mild case of Asperger’s Syndrome but he didn’t believe that had any role in her disappearance.

Kristine Lazar reported from outside the family’s Cheviot Hills home. She said police and a steady stream of detectives were there all day looking for clues.

The lead detective on the case told Lazar the Lopez twins had also not been seen since Monday. They had email contact with their father but when asked if they knew where Karimi was, he said they did not reply.

Karimi’s parents said their daughter, a recent high school graduate, had no history of depression or running away.

It was unclear if Karimi was able to explain to her mother why she decided to go on the last-minute road trip or what was behind the adventure.

Detectives said Karimi’s cell phone had been turned off making it impossible to track her movements.