SEATTLE -- King County Prosecutor Dan Satterberg has filed first-degree murder charges against a man accused of killing a woman whose body was found in the trunk of her car.

Satterberg said the victim, Jane Britt, 75, fought hard against her assailant "over a prolonged period of time" and was strangled by the attacker, who used a ligature and came at her from behind.

RAW VIDEO: Satterberg Announces Charges In Britt Case

Police last week arrested Joseph Njonge, 24, a certified nursing assistant at the Garden Terrace Care Facility, where the victim's husband is a resident.

Britt's body was found March 19 in her car that was parked at the facility.

"The solution to the crime itself started, tragically, at the time of the attack," he said, when Britt fought the attacker and suffered broken fingernails and defense wounds on her hands.

It was DNA from skin cells found under her fingernails that led to the suspect's capture, he said.

The DNA was tied to the suspect, one of several employees of the center who provided cheek swabs of their DNA.

Police said they found Britt's husband's Costco card inside Njonge's wallet but said that he denied killing Britt.

Police said the motive in the killing is still unknown. Njonge, who is originally from Kenya, worked for the care center’s parent company for four years and had passed state and national background checks.

Njonge is scheduled to be arraigned on April 17.