Carter W. Geyen and his friend, both 12, took their homemade slingshot to Bruce Russell Park in Roseville on Sunday to launch a baseball, rocks and even chunks of concrete.

Their device — a length of surgical tubing stretched between the tops of two posts — could fire items high enough into the air that they left craters in the ground when they landed.

But when the boys tried to launch a 5-pound rock, the fun turned lethal.

The rock shot a short distance into the air, then fell directly on Geyen’s chest. He was pronounced dead at Hennepin County Medical Center in Minneapolis.

Geyen grew up a block and a half away from the playground and visited the park almost every day, said Kelly McDyre, who spoke Monday on behalf of the family.

McDyre said a doctor determined the boy died of blunt-force trauma to the heart.

“It was instant,” McDyre said. “The pain from the loss of Carter is overwhelming. (His family’s) lives and dreams have been shattered, and their hearts are broken.”

In his obituary, the family said their son died doing what he loved most — playing.

“There was nothing suspicious; it was just a tragic accident,” said Roseville Police Chief Rick Mathwig.

Police said it appeared the boys secured the tubing and its nylon sling between 9-foot-tall posts on the playground.

They fired the slingshot by one boy stretching the tubing to the other boy lying on the ground between the posts. A nylon sling — described by police as a modified fanny pack — held the projectiles. The boy on the ground then let go, shooting the projectiles into the air.

When police arrived about 3:30 p.m. at the park on Roselawn Avenue near Lexington Avenue, they found a dozen craters in the ground.

An autopsy was scheduled for Monday, Mathwig said.

Hamad Jaman, 18 of Roseville said he arrived at the park to play tennis apparently moments after the accident. He said he saw a boy run out of the park and another boy who appeared to be sleeping near the playground equipment.

Jaman said he and a friend played tennis for a few minutes but became concerned when they realized the boy on the ground had not moved.

“We made our way over to the kid and tried to get a response from him. There was no movement, no nothing,” Jaman said.

The two moved the boy’s arm from over his face and saw his lips were blue.

“We checked his pulse,” Haman said. “There was nothing.”

Jaman and his friend called 911. A man nearby rushed over, Jaman said, and tried CPR.

Jaman said he saw a rock lying next to Geyen that was “bigger than his chest.”

He said rescue workers arrived and tried to revive the boy for more than 45 minutes.

Geyen’s playmate returned to the park with his parents, Jaman said, and talked with police.

“I wish we were there earlier to have them stop playing the game,” Jaman said.

Geyen is survived by his parents and an older brother.

A funeral was scheduled for 11 a.m. Thursday at Roseville Lutheran Church, 1215 W. Roselawn Ave., adjacent to the park.

John Brewer can be reached at 651-228-2093.