No charges will be filed in the case of a UPS driver who was mauled by a pack of pit bulls in Puyallup last year because investigators say they cannot find witnesses that could talk about the dogs' prior behavior.

The Pierce County Prosecutor's Office said Monday the prior incidents are necessary to charge a crime.

At a hearing last November to determine the fate of the dogs, driver Kevin Backlund testified in court about the September 2017 attack and showed graphic photos of his mauled arms and legs. Unable to sit because of his injuries, the 59-year-old recounted how he arrived at the east Pierce County property in September.

Related: UPS driver's 911 call: 'I'm being attacked by five pit bulls'

Listen: 911 call of UPS driver in dog attack

Related: UPS driver manages to call 911 while being mauled by 4 dogs in Puyallup

Related: Evidence photos reveal grisly scene of pit bull attack

As UPS drivers are accustomed to doing, Backlund testified he honked his horn, rattled the fence and looked for any dangers.

Seeing nothing, he entered through the unlocked gate to deliver the package.

Seventy-five feet onto the property, four pit bulls surrounded him.

"They attacked all at once," Backlund said. "Not one, and two and three, all four at once. "

"I was in a fight for my life."

Backlund retreated onto a flatbed trailer trying to escape the dogs. He managed to call 911.

Orting Valley Fire and Rescue officials testified when they arrived, they found Backlund "on top of this trailer covered in blood, he had a dog hanging off his leg."

Firefighters rescued Backlund by ramming the gate with his truck so Backlund could jump into the passenger side window.

The hearing examiner ruled in December the dogs were to be euthanized.

Backlund is suing the dogs' owner, according to his attorney.