A man walks into an Illinois church and opens fire, killing a pastor and wounding others.

An Alabama man goes on a shooting rampage, killing 11 people before committing suicide.

A German teen kills 15 people, including several students, teachers and a passerby, before turning his gun on himself.

The initial reactions to the killing sprees of the last few days are often ones of disbelief and bewilderment. But more often than not there were signs that were missed or not acted upon, says Dr. Keith Ablow, a forensic psychiatrist and a FOX News contributor.

“The way in which something like this occurs can be multifaceted,” Ablow says. “Often this happens when someone fails in his life to see a future for himself. It’s the point at which someone considers there to be only darkness up ahead and irrationally feels that all the actors need to leave the stage with him.”

Rampage killers also tend to feel powerless in their lives, Ablow said. Often, they suffered abuse in their childhoods that left them feeling completely overpowered. The massacre is the killer’s rush to be visibly powerful in the most horrific kind of way.

The loss of a job, a home and a relationship are often the triggers for these thoughts and the resulting actions, Ablow said.

“A lot of triggers are deep in people’s life stories. As for what moves someone to actually commit violence today as opposed to some other day, losses are a trigger."

Although the motivations may be different, spree killings don’t happen out of the blue. There are often signs that people don’t see or want to see.

“Sometimes people will come right out and say that they are at the end of their ropes and considering violence, and because many people don’t want to believe a psychological thriller could be unfolding, they tell themselves ‘no, it couldn’t be so,’” Ablow said.

Families and friends should be concerned about people who say “they have nothing left to live for,” and for signs that people are cutting themselves off from friends and loved ones, creating a “social isolation” for themselves.

Ablow suggests being a bit of a detective. Look for signs ... Is there not only depression, but a sudden interest in weapons or violence?

"I think there needs to be a willingness to believe that something’s going to happen, and a willingness to respond,” he said.

Ablow advises people not to be afraid to reach out. Social workers, psychologists and psychiatrists are available to help people handle a crisis, he said.

“It is possible that real healing can take place. There are a lot of medicines that can help people deal with depression and anxiety. And people need to be willing to use law enforcement.

"Even though it’s complicated and may complicate your life to involve law enforcement, people shouldn’t be afraid.”

Ablow said the recession can be the last straw for potential rampage killers.

“I think it’s critical as the economy continues to suffer to put on our radar to detect people who are over the edge,” he said.

“Look for people who may be violent. There are people working through deep psychological issues -- the loss of a job, the loss of a home, the financial discord that occurs in marriages from the loss of a job. It’s why family members need to be more in tune with what’s going on with the people around them.”

And just as the reasons for the killings tend to be multifaceted, so do the killers’ choices of who should die and who should live.

In the case of school or office shootings, the motive can be revenge.

“What motivates killers can be very different,” Ablow said. “For one person, the killings may represent a projection [of] what they’re feeling inside at that moment. For another person, it can be revenge.

"Of course, it’s completely irrational to think that you’ll make something right by perpetrating violence upon those around you.”

When someone chooses to take the lives of family members, as was the case in Tuesday’s Alabama slayings, the killer often feels as though his loved ones can’t live without him.

“Killers often have irrational beliefs about their importance to the dramas around them,” Ablow said. “They may feel their families can’t survive without the financial stability they’ve been providing.

"Depression, when severe, deprives people of the ability to feel empathy for others. There’s also a delusional quality to depression. People feel like there’s a blackness descending upon them and that there’s no hope for anyone.”