CHICAGO (AP) — The twin 5-year-old boys had just climbed out of the bathtub and into their pajamas the night before the first day of kindergarten when a relative’s handgun they’d found went off.

The bullet struck Jonathan Jackson in the stomach. Less than an hour later, after being rushed to Mount Sinai Hospital, he was pronounced dead at 11:07 p.m. Monday.

On Tuesday, family members gathered at the house in Chicago’s Lawndale neighborhood to grieve. While none of them were in the boys’ room when the gun went off, a great uncle said the twins must have been sharing the gun, as they did with everything.

“They never had a fight over anything,” said Bruce Prince, who lives in the two-flat where the shooting occurred. “If they had a bag of chips, they shared it. They don’t fight for it.”

WATCH NBC CHICAGO COVERAGE OF THE KILLING:

Police said the shooting occurred at about 10:40 p.m. The boys’ grandmother told officers that she heard a single gunshot and then one boy ran out of the room, followed by his brother.

“Jonathan kind of crawled,” said Prince, who said he was outside at the time and did not hear the shot.

Prince said family members have not been told by police definitively which boy fired the weapon. Police spokesman Roderick Drew said detectives believe the gun went off accidentally, but that they were trying to determine the exact circumstances that led to the shooting. Because of the other boy’s age, his name was not released.

Drew said police were investigating who owned the handgun, which was recovered at the scene, but declined to comment further.

Meanwhile, the boys’ great grandmother, Lucille Prince, said relatives believe the gun belongs to a 17-year-old uncle of the twins and that late Tuesday afternoon he was brought to the police station for questioning. Drew would not comment.

Bruce Prince said that he is certain the boys did not understand it was a real gun, and thought it was a toy.

He also said that it wasn’t clear which boy fired the gun, but that it would not surprise him if Jonathan was showing his brother the weapon.

Born 2 minutes and 39 seconds before his brother, Jonathan, Prince said, “was the natural leader.”

Jonathan helped his brother get dressed and tie his shoes, because he’d already learned how to tie his own, Prince said.

Now, he said, the surviving twin is worried about his brother, but he is wondering when the ambulance that took him away will bring him home.

“He don’t understand the concept of life and death,” said Prince. “He thinks Jonathan is in the hospital.”

Copyright 2010 The Associated Press.