FERNLEY, Nev. (NewsNation) — A coroner ruled that Naomi Irion died from multiple gunshot wounds to the chest and head before being buried in rural Nevada as authorities upped charges against her alleged killer.

The 18-year-old was kidnapped from a Walmart parking lot while on her way to work in rural Fernley, Nevada on March 12. She was reported missing by her family shortly after.

Authorities arrested Troy Driver, 41, in connection with her kidnapping in March. Lyon County District Attorney Stephen Rye filed the amended criminal complaint Tuesday adding first-degree murder and other crimes to the kidnapping charge already facing Driver.

Driver is accused of kidnapping Irion from the Walmart parking lot in rural Fernley and killing her on or before March 25. Four days later, investigators acting on a tip found her body in a grave.

In addition to burying Irion’s body, the new complaint says Driver disposed of tires from his truck and destroyed or hid her cellphone in an effort to eliminate evidence that might help lead to his arrest.

Since his arrest, Driver has been held on $750,000 bail in the Lyon County jail in Fernley, about 30 miles east of Reno.

He’s now accused of first-degree murder with the use of a deadly weapon, first-degree kidnapping, robbery, burglary of a motor vehicle and destroying evidence, according to the new complaint Rye filed Tuesday in Lyon County’s Canal Township Justice Court in Fernley.

The complaint said Driver shot Irion in or near a rural portion of northern Churchill County northeast of Fernley, where he took her “for the purpose of committing sexual assault and/or purpose of killing her.”

It said he entered the 1992 Mercury Sable Irion was sitting in at the parking lot in Fernley with intent to commit grand petit larceny, assault or battery, then drove it away with her inside against her will.

Diana Irion, Naomi’s mother, told NewsNation her family has seen a longer cut of that video that shows Driver approach Naomi’s driver’s side door and get in without a struggle.

Her car was found three days later at a nearby paint production plant.

Reports from 1997 in the Ukiah Daily Journal in California detailed Driver’s arrest in connection with the murder of a 19-year-old. Driver was 17 at the time of the crime and pleaded guilty to multiple counts of robbery, burglary, use of a firearm and being an accessory to murder after the fact, California court records show.

He was not charged with or convicted of murder. Two others also were arrested in connection with the same incident.

Irion’s family described the young woman as a magnetic and genuine person who usually maintains regular communication with her friends and family. A diplomat’s daughter, Irion moved to the U.S. from South Africa in August to gain independence and save up for a place of her own some day, family said.

Driver was set to appear in court Tuesday for a pretrial status hearing, but a judge continued that hearing until May 10, according to the Associated Press.

NewsNation affilate KLAS and the Associated Press contributed to this report.