Two Utah Police shot and killed Danielle Willard, 21, outside an apartment complex on Nov. 2, 2011. Facebook/Justice for Danielle The parents of a 21-year-old woman who was shot in the back of the head by two Utah police officers have filed a wrongful death lawsuit.

West Valley, Utah police have said they thought Danielle Willard was buying drugs when they killed her in an apartment complex, the Salt Lake City Tribune has reported.

She tried to back her car into them when they approached her, police say.

The suit claims she was "shot in the back of the head, assassination style" and that it was "without justification" and "unrelated to any legitimate law enforcement purpose."

The death of Willard set off a public firestorm over the West Valley, Utah's narcotics department. The FBI began investigating the department, as did the city itself, The New York Times reported. West Valley officials found that in some cases officers on the narcotics squad mishandled evidence.

Willard, who struggled with heroin addiction, had gone to Utah for rehab, her mother Melissa Kennedy told The Times. She was unarmed when she died.

Her mother says part of the reason for filing the lawsuit is to find out exactly what happened to her daughter.

"It's the only way that people are going to know what happened to her and why," Kennedy told the Tribune.