SPOKANE -

An 18-year-old Fairfield, Wash. man is dead and his female passenger in critical condition after their car hit a tree early Saturday in south Spokane County, according to sheriff's detectives.

Investigators say the crash happened at about 1:00 a.m. Saturday on the Palouse Highway near Dunn Road.

Friends say Levi Larrison and his girlfriend, 18-year-old Tiffani Lewis, were on their way back to Waverly and Fairfield when the crash happened. Investigators say the car went off the road and slammed into a tree at a high rate of speed.

Levi was pronounced dead at the scene. Tiffani was airlifted to Sacred Heart in critical condition, but family members say her condition worsened and plans are now in place to donate her organs.

According to Tiffani's younger sister Maylissa, the two recent Liberty High School graduates were very much in love with each other.

"It was her first love and he wanted to marry her," said Maylissa. "They always said if they were going to die - they wanted to die together."

Tiffani was a cheerleader at Liberty with a love for sports, dancing and signing. Friends say Levi was an accomplished athlete, playing both basketball and football for Liberty.

Classmates remember Tiffani as the prom queen, and Levi her prince.

"She was sweet and was everything to everybody, always cheerful and happy," said Maylissa. "[Levi] was funny. He always knew how to make people laugh - he was fun."

Now the Spokane County Sheriff's Department Traffic Unit has the task of determining exactly what caused the crash. Investigators did say both teens were wearing their seatbelts.

For the students at Liberty who knew the two, school officials say grief counselors will be on hand Monday to help deal with the loss.

Family members say memorial services will be held later this week for both teens.