(CNN) -- The identities of two women involved in a horrific car crash in Arizona last week were mixed up, with one family receiving news that their daughter had died when in fact she was undergoing treatment for her injuries while another family kept watch at her side for six days.

Officials with St. Joseph's Medical Center and the Arizona Department of Public Safety apologized for the mistake Monday, saying they were "acting on the best information" available at the time of notification.

Marlena Cantu, 21, was killed in the crash July 18 when the SUV in which she was riding blew a tire and rolled several times. Four others were in the vehicle: a male driver and male occupant, and two other female occupants -- one of whom was 19-year-old Abby Guerra, described by friends and family as Cantu's best friend.

The five young people were driving back to Phoenix from a trip to Disneyland in California.

"As doctors struggled to save this patient's life, we could not readily tell the patient's identification," Suzanne Pfister, vice president for external affairs at St. Joseph's, told a news conference Monday. "We interviewed family members that evening and asked for any discernible information that could help us make that positive identification," such as bone breaks, scars or other marks.

"From the information that was provided us on that evening, we believed the patient was Marlena Cantu," Pfister said. "DPS communicated that information to the family."

Guerra's family, meanwhile, was told she was dead.

Pfister said the mix-up was revealed Saturday when the medical examiner compared dental records and determined the patient -- whom the Cantu family had been praying for at her bedside as she underwent multiple surgeries -- to be Guerra.

"We know this is an extraordinarily challenging situation and our hearts and our prayers go out to both families," Pfister said.

Sgt. Kevin Wood, a spokesman for DPS, described a "chaotic situation" in the emergency room the night of the wreck as the hospital received all four trauma patients from the collision scene. At least one person had been ejected from the SUV.

"(Authorities) tried for quite some time -- something close to five hours -- to figure out who this one female patient was," Wood said, emphasizing it was not a determination that was made within a matter of minutes.

Pfister said the injuries to Guerra were so severe that she was unrecognizable to family and friends.

She said the hospital, which treats about 3,000 trauma patients a year, does not conduct fingerprinting or take dental impressions in an emergency situation "because the overwhelming emphasis is on saving the patient's life."

Nancy Colon, a friend of the Cantus, told HLN's "Prime News" Monday night that the family is "heartbroken."

Colon said the family was taken into a room at the hospital on Saturday and told that the dental records taken from the female victim at the morgue did not match Guerra's. They were then questioned about the number of Marlena Cantu's ear piercings, her molars and past surgeries.

Cantu had her appendix removed in elementary school, Colon said the hospital officials were told.

"They checked the girl in the ICU and she didn't have a scar," she said.

The family was then told, "I'm sorry, but that's not your daughter," according to Colon.

Pfister declined to provide information about Guerra's condition Monday, citing the family's request.

"Our focus is on healing Abby," she said.

The tragic mix-up is similar to another incident in 2006. The family of 22-year-old Laura VanRyn spent five weeks at the bedside of a woman they believed to be her daughter as she struggled to recover from injuries sustained in a car wreck that killed five others. When she awoke from a coma and was asked her name, she replied, "Whitney Cerak," an 18-year-old who was believed to have been among those killed in the wreck. The two women had blond hair and similar body types.