We always want answers.

NFL linebacker Jovan Belcher, 25, shot to death the mother of his child, and then shot himself in the head in the middle of the afternoon Saturday outside the team's Kansas City practice facility.

The day before, a man killed his father's live-in girlfriend and then burst into a Wyoming college classroom where his father taught and killed him, police said. The man then ended his own life.

Experts who spoke with CNN say there are about 1,500 murder-suicide incidents in the United States every year. In comparison, there were 14,612 murders in 2011, the FBI says, and 38,364 suicides in 2010, according to the Centers for Disease Control and Prevention.

The 1,500 figure is questionable, experts caution, because there are no official statistics on this kind of crime -- the FBI doesn't keep track, and police classify murders in different ways.

This lack of certainty often amplifies the frustration people feel when loved ones are wrenched from them so violently. And it makes it even tougher to understand when the violence is wrought in public.

Here's what we know from research about murder-suicides:

-- Fathers who kill their children, then themselves, are usually older than the mothers.

-- Older couples in murder-suicides are more likely to have medical illnesses.

-- Older men are more likely to kill themselves.

-- Younger couples are more likely to have a history of verbal discord.

-- Firearms are the favored weapon.

-- Victims are overwhelmingly female.

-- Estrangement is typically the biggest contributing factor.

Leaving the living to question everything

Fourteen years ago, Jamie Righter's brother was estranged from his former fiancee. The couple had trouble conceiving and were seeking fertility treatments. She had a relationship with another person and became pregnant, Righter told CNN, and her brother was incensed. Fueled by alcohol, the brother killed the ex-fiancee and then himself.

"My brother was never violent or abusive a day in his life," she said. "Nobody ever expected this. My brother was the kid next door. I have spent years trying to understand."

Righter and her mother formed the Community Awareness and Support Center in Oregon specifically for family members of people who had committed murder-suicides.

"I feel like there's nobody who is telling their story," she said. "I feel like all you do is struggling, and you go back and you go back, and what could we have seen or done? The living are faced with what is left.

"You are called 'murderer's family.' People harass. They talk. It can be very hard."