(CNN) -- 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 burst into a Wyoming college classroom, police said, and killed someone he knew, and then killed himself.

Usually, such tragedies are shocking.

Experts that CNN spoke with say about 1,500 murder-suicides happen in the United States every year.

And even that number is questionable, they caution. There are no credible 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.

There are generally two types of people whose behavior could set off alarm bells as possible attackers.

One is a "middle-aged man who is recently separated or facing pending estrangement from an intimate partner and who is depressed and has access to firearms," writes Dr. Scott Eliason. "The other is an "an older male who is the primary caregiver for a spouse who is ill or debilitated, where there is a recent onset of new illness in the male, depression and access to firearms."

That's according to a study Eliason published in the Journal of the American Academy of Psychiatry and the Law.

Sometimes, people want to kill themselves because they perceive someone has stopped loving them. The person who is perceived as withholding affection becomes a target.

"A person who is miserable about the loss of affection in their life achieves compatibility again with this person by execution," said Dr. Frank Campbell, the executive director of the Baton Rouge Crisis Intervention Center in Louisiana. He's been studying suicide for more than two decades and travels around the world to investigate suicide scenes.

"When one commits murder-suicide, they create acceptable consequences to them -- they are now with that person in a way they can control," Campbell said.

Because murder-suicide statistics aren't readily available, it's not possible to be sure how many happen in public spaces, like in Kansas City and Wyoming.

But choosing a public space, or at least doing it in front of other people, is a way to make a "major statement," said Dr. Nadine Kaslow, a professor in the Department of Psychiatry and Behavioral Sciences at Emory University in Atlanta.

"Often these are impulsive acts, too," she said. "There's just no way to know what was going through someone's head."

Public murder-suicide can be an act of communication, Campbell said. It's a way of acting out frustration and pain that cannot be conveyed in another way.

And though it's typically between two people who know each other, the label murder-suicide includes mass killings, after which the killer or killers end their lives.

Experts mention infamous Columbine High School killers Eric Harris and Dylan Klebold; or Seung-Hui Cho, the college student who killed 32 others before taking his life at Virginia Tech in 2007.

There's a lot of looking in the rear-view mirror, too. The spider web of people who are affected by a murder-suicide -- friends, family, even the community that sees it in the news -- are burdened to look for hints they might have missed.

So know this: Sometimes, people contemplating suicide warn us, said Dr. Charles Raison, a CNN contributor and an associate professor of psychiatry at the University of Arizona.

"People who kill themselves often will tell someone ahead of time," he wrote for CNN after the suicide of filmmaker Tony Scott. "Any such communications should be taken with utmost seriousness, and all efforts should be made to keep the person safe and get him or her to appropriate treatment immediately."

™ & © 2012 Cable News Network, Inc., a Time Warner Company. All rights reserved.