A California probation officer and her two children are dead in a presumed murder-suicide, after her husband was shot and she engaged in a five-hour standoff with police.

The horrifying incident unfolded early on Saturday morning on a quiet block of suburban ranch homes in Ontario, California on the eastern outskirts of Los Angeles.

At around 2.37pm, a man in the home called 911 for medical assistance, saying that he was bleeding.

On arriving at the scene, Ontario Police officers and paramedics learned that the man had what appeared to be a gunshot wound. As they were speaking to him, his wife emerged from the home armed with a handgun, according to police.

The horrifying incident unfolded early on Saturday morning at this home in Ontario, California

Two officers on the scene opened fire. Officials did not specify whether the woman was struck by police gunfire.

The woman, an officer with the San Bernadino County Probation Department, retreated back into the home, and the officers learned that she was likely barricaded in with two children, a teenage girl and an elementary aged boy.

Additional units responded to the scene, including the Ontario SWAT team and the Crisis Response Team.

What followed was a five-hour standoff, as hostage negotiators tried desperately to convince the woman to surrender peacefully.

Finally, officers made entry to the home and discovered a gruesome scene. The woman and both children were dead.

The husband was taken to the hospital with possible life-threatening injuries, according to authorities.

No police officers were injured in the incident. The names of those involved are being withheld pending family notifications.