Two youths who served alcohol at an unsupervised high school party where a boy collapsed and later died were sentenced to community service, fined and placed on two years of probation Tuesday during a somber hearing in which the boy’s mother blamed them for her son’s death.

Patrick “P.J.” and Alexandra “Ali” Gabrielli, 18 and 20 respectively, at whose Orinda home the party was held, listened gravely as a Contra Costa County prosecutor read a statement from the mother of 16-year-old Joseph Loudon, the high school sophomore who died after attending the May 23 party.

Marianne Payne said she believed her son’s death was the “result” of the Gabriellis’ “actions and inactions” that night. Although the siblings initially waived their Miranda rights and spoke to police, Payne said they later refused to cooperate with investigators.

“The silence has left us without answers,” wrote Payne, who said through the prosecutor that she was too emotional to appear because it was the nine-month anniversary of her middle son’s death. “There has been built a great wall of silence.”

Loudon, described by friends as witty, athletic and spiritual, collapsed in a hallway after spending less than two hours at the party. Witnesses described him as bluish and said he did not appear to be breathing.

A girl revived him with CPR, but no one called 911. Most of the partygoers, including the Gabriellis, did not witness the collapse, investigators said. Loudon told friends he would be fine, and they helped him to a bedroom, where he later vomited and aspirated.

An autopsy revealed that Loudon had a blood-alcohol level of .03%, the equivalent of about a beer. Medical examiners listed the cause of death as “undetermined” and suggested that he had suffered from a previously undetected heart ailment.

The Gabriellis’ mother and stepfather were out of town the night of the party. P.J. Gabrielli was then a junior in high school, and his sister a sophomore in college. Loudon, who had told his mother he was going to the movies, lived across the street from the Gabriellis, and he and his two brothers grew up with them.

Payne said she wished she could have foreseen “future dangers lurking below the surface of our seemingly idyllic community,” an affluent, wooded town bordered by the Oakland and Berkeley hills.

“Little did I know,” she wrote, “that they were right across the street.”

The Gabriellis pleaded "no contest" to a misdemeanor violation of providing alcohol to minors, a plea that is treated as an admission of guilt for sentencing but cannot be used against them in a civil lawsuit.

They were each fined $1,000, told to perform 200 hours of community service and warned that they could face jail if they drank alcohol or violated any law during the next two years.