One young man had a history of depression and drug abuse. Another was said to closely follow the Columbine case and reject help from counselors. And a fight at school appears to have provoked a third.

Three shooting rampages in a one-week span have refocused attention on troubled youth: a 19-year-old man opened fire at a Nebraska mall, killing eight people and himself; a 24-year-old man killed four people at a megachurch and a missionary training school in Colorado and then killed himself; and now police are searching for gunmen who wounded six students at a school bus stop in Nevada, following a fight about a girl.

"There is this copycat phenomenon with highly publicized homicides, multiple shootings and other suicides," says Kathryn Seifert, a forensics psychologist and author of "How Children Become Violent." She says people who are already troubled or considering violence might say, "'Well see somebody does it and gets a lot of publicity. I'm going to do the same thing.'"

But Seifert says she believes the common denominator is that these men had troubled pasts.

The mall shooter was reported to be a drug user with a history of depression. He was a ward of the state after spending time in a treatment facility for threatening to kill his stepmother. He was a high school dropout who had recently lost his job at a fast-food restaurant and broken up with his girlfriend.

The church gunman had apparently sent hate mail to the mission center and posted online threats in between his two deadly shooting sprees about killing Christians. Police said they believed Tuesday's shooting at a school bus stop was linked to a fight at a high school earlier in the day.

While there is no single profile of a shooter, the warning signs of violence generally appear in childhood, says Seifert. It can be difficult for parents to determine what goes beyond normal child behavior, but experts say to look for a combination of the following over a period of time.

If the child is violent or suicidal, seek help immediately, says Seifert. Otherwise, seek help if the problems interfere with the child's home or school life for more than a month and the parent is unable to resolve the issue.

Poor coping mechanisms

A troubled child may be unable to cope with frustration, disappointment, or stress, manifesting into anger or severe depression, says Margaret Ross, President and Founder of the Kamaron Institute. "The reaction is larger than the situation and it's regularly larger than the situation," she says. "There are no small deals. There's only big deals."

Drug/alcohol abuse

If your child becomes a substance abuser, that might not be his only problem.

Unsafe access to firearms

"If your child has made a specific weapon-related threat and has unsafe access to a weapon, that is known to be a very dangerous combination," says Dan Gross, co-founder and CEO of PAX, a nonprofit gun violence prevention organization. "The key ingredient is always the weapon. That's what elevates an idle threat to one that poses much more serious risk."

Exposure to violence

Your child might be exposed to violence in the home or community. "Domestic violence is really a strong risk factor," says Seifert. "That becomes the children's model for problem solving."

Setting fires

If your child sets or attempts to set fire to his home, someone else's home, or his school, that's cause for concern about future acts, says Seifert. "There are theories that fire-setting is an indication that a child has been sexually abused," she says. "Certainly sexual abuse is a risk factor for being violent, but not that by itself."

Violent role models

Pay attention if your child idolizes other shooters, or has a fascination with violent behavior, says Lisa Pescara-Kovach, professor of educational psychology at the University of Toledo and author of "School Shootings and Suicides: Why We Must Stop the Bullies."

Assault

Be concerned if your child harms animals or assaults other people, particularly a small child or authority figure, says Seifert. "The more assaults a young person has committed in the past, the more predictive that is that they are going to assault someone in the future." If a child has no remorse for hurting someone, that's a warning sign.

Family history of criminal behavior

Criminals are generally unable to teach children right from wrong, says Seifert. An incarcerated parent can be a risk factor if that parent is sending the message that criminal behavior is OK, she says.

Depression

If your child is isolating himself, puts himself down, and talks about feeling hopeless, like the world is out to get him, these are all signs that he is depressed. Males are more likely to act out their depression in a violent way than females, says Ross.

Bed-wetting

This is a minor risk factor if your child is over the age of 5 or 6 and wets the bed, says Seifert, because bed-wetting has been associated with early childhood abuse, neglect and exposure to domestic violence. Considering 5 to 7 million children 6 or older suffer from nighttime bed-wetting, according to WebMD, parents should make sure to look for other signs.