The Amherst High School sophomore who died Sunday morning had breathing problems that developed into pneumonia and sepsis, her father said Monday.

Chelsea Oliver’s family, friends and doctors still aren’t sure what caused the 15-year-old’s death and are awaiting autopsy results and testing for swine flu.

Her father recalled her Monday as “a beautiful girl who made everybody around her feel better.”

“She would be the one person who would be able to get us through this,” Adam Oliver said.

Chelsea, of Snyder, the second oldest of five children, was admitted to Women & Children’s Hospital on Oct. 25.

Amherst School Superintendent Laura K. Chabe and a spokesman for the Erie County Health Department said they were prohibited by federal privacy laws from discussing specifics of the illness.

Given concerns about the spread of swine flu, school officials conferred with Health Department representatives after Chelsea’s death to make sure they were following proper procedures to limit the spread of both seasonal and H1N1 flu. It was determined that the procedures have been proper, Chabe said.

“We are very sad to inform you that following a brief illness one of our high school students passed away Sunday morning,” Chabe said Sunday in automated phone calls to about about 3,000 parents, faculty and staff in the school district. “It’s a very sad day in our district and community, and our condolences go out to the family and friends of our student.”

She also said school officials have been in touch with Health Department officials “and have been assured that it is safe for your children to be in school.”

Chabe on Monday morning described the mood of the district as “very solemn, as you would imagine.”

Chelsea’s friends gathered at Amherst High School and online to remember their classmate as a talented singer and a gifted artist.

Students stopped by a second- floor classroom throughout the day to write messages on large posters that will be given to her family and to admire a piece of art that Chelsea had painted, said Mike Sciandra, a friend who met Chelsea in the school’s Writers’ Guild last year.

“It was so hard. There were a lot of tears. She was such a beloved person,” he said.

A Facebook group titled “In memory of chelsea oliver” had 250 members early Monday evening. One photo on the page showed Chelsea playfully posing with a flower petal stuck to her nose.

Adam Oliver said his daughter, who liked dressing “alternative” and performed solo at the school’s spring concert, enjoyed the limelight but also “took the time to show she cared about people.”

“She touched so many people and was loved by so many,” he said.

Kevin Montgomery, a spokesman for the Erie County Health Department, said his office could only tell people to continue to take precautions to prevent the spread of flu, including frequent hand-washing and staying home after the onset of symptoms.

Fifteen school districts in Erie County last week reported increases in flulike illnesses among students, nine reported decreases, and five said they were holding steady, Montgomery said.

School districts hit hard by the flu are reporting absenteeism rates of more than 20 percent, Montgomery said.

When the flu is prevalent in a particular school, it generally spikes for about a week before receding, he added.

Two Buffalo students died of the H1N1 flu virus in June. Nationally, 292 laboratory-confirmed H1N1 deaths occurred between August and Oct. 10, according to the Centers for Disease Control and Prevention.