ABC News' Claire Weinraub and Tess Scott report:

Todd Francis is facing four years in prison, but the father of two says no prison could be worse than where he lives now.

This week, Francis stood in a California court and pleaded guilty to two counts of endangering a child, after a gun he owned and kept in his garage killed his daughter's friend.

Watch: ABC News' hidden camera experiment.

"I'm still in shock," Francis told ABC News' Diane Sawyer during January's special "20/20? report "Young Guns."

In June 2013, the day had been an ordinary one for Francis, a computer specialist, who had gone grocery shopping and left his teenage son in charge of his daughter.

Then he came home to a crime scene.

"I had groceries in my hand and you know. … It was like every police squad car was in my neighborhood," Francis said.

His daughter, Cierra, 9, and her 10-year-old friend Eric Klyaz had been handling his gun - a 9 mm pistol - when it went off. Eric was killed.

Francis said he kept the unloaded gun stored in a box in the garage with the ammunition kept separate. The prosecutor argued that the gun had been loaded.

Related: Parents say teaching young kids how to shoot prevents tragedies.

Francis said he got the gun after hearing about a number of home invasions occurring in San Diego.

"It just terrified me to think that, you know, somebody could break into my house, with us there, and hold us captive," he said.

California is one of 14 states that hold adults criminally responsible for storing guns unsafely. Massachusetts actually requires gun owners to lock them up.

A 2000 study by Johns Hopkins University found that in California, Florida and Connecticut, where there are felony penalties for leaving a gun accessible to a child, there was an associated 31 percent drop in accidental child shootings. Since then, Massachusetts and Washington, D.C., have passed similar laws.

Watch: Where should parents store their guns at home?

Last year, ABC News found at least 28 cases of adults facing criminal charges after a child accidentally shot and killed themselves or another child. In one case, the penalty was 55 years in prison.

"I feel horrible about what happened," Francis said. "I do feel accountable. … I thought about him like a son, like another family member."

He said his wife left him after the deadly accident and took the children with her. He has not seen his son or daughter since the day of the accident. He said he knew that his daughter thinks the gun shattered her life as well.

"Keep your guns locked," he said, "and don't assume that you're in control of your household."