Prosecutors have declined to press criminal charges against a 16-year-old girl who had possessed her driver’s license for less than two months when she struck and killed a pedestrian and her dog in Harpswell.

The teenager, Isabella Slocum of Harpswell, hit 75-year-old Rita Douglas and the dog Aug. 5 as they walked along Harpswell Island Road at about 6:40 a.m.

On Monday, Cumberland County District Attorney Stephanie Anderson announced the decision not to prosecute Slocum after the Cumberland County Sheriff’s Office and the Brunswick Police Department concluded months of investigation.

“The investigators did a very thorough job,” Anderson said in a written statement. “They reconstructed the accident and investigated whether speed, alcohol, drugs or driver inattention were involved. The driver stopped immediately and was fully cooperative, including turning her cellphone over to investigators. There was no criminal negligence or recklessness. This was a horrible tragedy, made even more so by the freakish nature of the circumstances.”

Slocum was issued a learner’s permit on Nov. 4, 2014, and her conditional driver’s license on June 18, after her 16th birthday in May. She had no prior driving accidents, according to the state Bureau of Motor Vehicles database.

Investigators found that Douglas, who lived nearby, was walking her dog along the northbound shoulder of Route 24, also known as Harpswell Island Road, as Slocum was driving north on her way to work in a 2012 Nissan Titan pickup truck.

The accident happened near a curve in the road, close to the intersection of Harpswell Island and Mountain roads. Harpswell Island Road has wide shoulders and the lanes are separated by a double-yellow line. The speed limit in that area is 30 mph.

Investigators found that the sun would have been in Slocum’s eyes as she crested a hill near where Douglas was walking, Anderson said in the media release.

Douglas’ husband, Robert Douglas, was working on his fishing boat on the morning of his wife’s death. He said later that he and his wife usually walked the dog, a Pomeranian named Foxy, together when he wasn’t working.

Rita Douglas was born in Germany and met her husband while he was based there in the military. The couple had planned to celebrate their 49th wedding anniversary, which was just days after her death.

Robert Douglas said in a brief phone conversation Monday that although he felt the sun could not have been in the driver’s eyes, investigators disagreed.

“The policemen said it was, so I have to take their word,” he said. “There’s not a lot I can do.”

An attorney for the Slocum family did not immediately return a phone call seeking comment.

Rita Douglas’ death added to an increased number of pedestrian fatalities statewide this year. Maine State Police said this month that 16 pedestrians had been killed as of Dec. 3, the highest number in at least a decade.

There were 13 fatal pedestrian accidents in both 2008 and 2009, the next-highest number in at least the past 10 years. Data from before 2005 were unavailable, the Maine Department of Public Safety said.