CAMBRIDGE — Karol Proctor’s voice broke with emotion as she told a packed courtroom that she failed as a mother.

Giving her victim-impact statement in Middlesex Superior Court, she said she wasn’t there to protect her son, Kelly, 19, from Daniel and Peter McGuane as he crawled away from them in the street, desperately hoping to find safety under a parked sport utility vehicle during a fight in Ayer on July 2, 2005.

Kelly’s friends pulled him, unconscious, from under the vehicle, and by the time he reached a hospital, he was dead.

“Having to crawl under a vehicle to escape his attackers must have been so painful,” she said, moments before Judge Diane Kottmyer sentenced the McGuane twins to five years in state prison at MCI-Cedar Junction for involuntary manslaughter. They were convicted last month.

The McGuanes, now 23, aren’t eligible for parole, but they’re getting credit toward their sentence for the two years they’ve already served while waiting for their trial and sentencing.

Prosecutor Kate MacDougall asked for a 10- to 12-year sentence, but defense attorneys Stephanie Page and Edward P. Ryan Jr. countered with a request for 30 months to serve and five years probation.

Kottmyer, however, stuck to the outer limits of guidelines for involuntary manslaughter, which call for a three- to five-year sentence.

She ruled on the McGuanes separately, but reached the same sentence for each.

The men surrendered to police two summers ago only because they realized their alibis weren’t holding up, then Peter attacked Kelly’s character, said Kottmyer.

“I consider that a second wrong of immense character,” she said.

A report issued by the probation department indicates Daniel understands how his conduct contributed to the tragedy, said Kottmyer.

On the streets of Ayer, several people expressed shock at what they said was too light of a sentence.

“It’s heartbreaking,” said Molly Merrow. “They shouldn’t be walking the streets even in five years. They should be behind bars for a long time.”

But Mark Pasciak said he’s glad the sentence wasn’t harsher.

“They were young kids,” he said. “They did something stupid. I’m glad to see their lives aren’t ruined. Justice won’t come from ruining two young lives.”

Daniel broke down in tears as Page read a letter from Peter to Karol apologizing for her son’s death.

“I can never apologize enough, and even though my apologies can’t bring Kelly back, I’ll never stop apologizing,” the letter reads. “I pray that one day you find it in your heart to forgive me.”

Karol left the courtroom without comment, but later issued a statement, which read, in part, “My son, Kelly, was the man of our house who looked after me and his sister, Kamara. We loved him with all of our hearts, and he was our best friend This should not happen to any family, any mother, any sister. Nothing will convince me that the beating of my son, Kelly, was anything but an intentional act. The McGuanes could have stopped, but they did not. Today’s sentencing will never bring my son back, and for that reason it will never be enough ”

Family friends were dismayed at the sentence and remain angry at medical examiner Dr. William Zane, whose autopsy results not only reached the wrong cause of death, but misstated Kelly’s race, weight and height.

The McGuanes were indicted on first-degree murder charges based on Zane’s report that indicated Proctor died from swelling and hemorrhaging of the brain, which are signs of a severe beating.

Defense attorneys Ryan and Page asked for a second opinion in January, and medical examiner Dr. Elizabeth Bundock, a neuropathologist, disagreed with Zane, who later admitted he made a mistake.

She suggested Proctor suffered a fatal brain concussion in which electrical impulses from the brain stem to the brain are disconnected, which stops critical life functions, so the Middlesex District Attorney’s office reduced the charges to manslaughter.

“I think it’s a joke,” Darryl Brown, a family friend of the Proctors’, said outside the courthouse. “The incompetence is mind-boggling — the district attorney, the coroner’s office — if they can’t get Kelly’s race correct, how could the jury return anything but involuntary manslaughter?”

The McGuanes’ supporters are also upset at Zane, insisting that if he hadn’t made a mistake on the cause of death, the twins wouldn’t have been indicted.

“The outrage of this case is if the medical examiner and prosecutor had done a thorough investigation, the charges would have been dropped,” said Ryan.

Kottmyer said she recognizes the sentence can’t ease the pain for Proctor’s family but also believes the McGuanes are remorseful.

“I believe they would do anything to go back and turn that situation around so the altercation never happened and Kelly Proctor never died,” she said.

Lowell Sun staff reporter Rob Yutkins contributed to this story.