A 9-year-old boy who collapsed at the Boys & Girls Club of the Harbor Area has died, Costa Mesa firefighters said Friday.

Myles Thames fell to the ground at 4:30 p.m. Thursday while shooting hoops on the club's basketball court, according to Battalion Chief Fred Seguin.

An autopsy performed Friday morning determined that Myles suffered a cardiac arrest as a result of hypertrophic cardiomyopathy, a thickening of the heart that makes it more difficult to pump blood, coroner officials said.

Titus Hasson, Myles' cousin who lives with the family, said when Myles was younger he had occasional health problems but hadn't suffered any in the past two years.

Boys & Girls Club staff members called 911 four minutes after he collapsed, and when the fire engine arrived, one staff member met firefighters saying, "Hurry, I believe he stopped breathing," Seguin said.

Myles' mother was at the club when firefighters were there, he said.

The boy was taken to Hoag Hospital and pronounced dead.

Myles played several different sports, including football and basketball, and wanted to sign up for soccer, said Hasson, a Newport Harbor High School senior.

The day before Myles collapsed, the two were wrestling and playing at home.

"That's the thing," Hasson said. "He didn't have any problems."

Myles was a student at Kaiser Elementary School in Costa Mesa, said Newport-Mesa Unified spokeswoman Laura Boss.

"It's just another tragedy," Boss said. "Just an absolute tragedy. I'm so proud of the staff at the school and everyone who's pulled together to support these folks."

Grief counselors met with several Kaiser students and staff Friday morning and were set to go to the Boys & Girls Club in the afternoon, she said.

Myles' two siblings attend Ensign Intermediate School and Newport Harbor High School.

"He was a happy kid," Hasson said. "Playful all the time. It's hard to talk about. He was just joyful all the time. Always liked to play, never did anything wrong. It doesn't make sense."

Hasson said he shared a bedroom with Myles, and things don't feel the same with him being gone.

"It's weird to come home and he's not there anymore."

lauren.williams@latimes.com; joseph.serna@latimes.com

Twitter: @lawilliams30 and @JosephSerna