DENVER -- Teens who witnessed Tuesday afternoon's shooting at Deer Creek Middle School may be deal with sadness, fear and denial for months or years.

"The research shows that up to 77 percent of children and teens exposed to a school shooting can develop PTSD," according to Dr. Dianne McCallister, the chief medical officer at Porter Adventist Hospital.

Teens may withdraw or want to avoid talking. Symptoms may surface months or years later.

Studies show that children with greater family support and less parental distress over the event have lower levels of post traumatic stress in the future.

"We, as parents will model ways to cope to our children, and so we need to be sure and put on our own oxygen mask first - in other words, you need to deal with your own feelings in healthy ways in order to help your children through theirs, " said McCallister.

Here is her recommendations for helping teens cope:

Maintain family routines

Making a safe space for your children to talk about their feelings. Make it ok for them to express their feelings, whatever they may be.

Limit exposure to the coverage of the event. It may feel like it's happening again if they see it on TV.

If you are having difficulty yourself, find friends or a professional to help you process your own feelings.

Minimize family conflict. A calm safe environment at home will give you all comfort.

"This may be difficult to do, since one of the responses to trauma, especially in older children and teens is to become impulsive or aggressive as an expression of these events," said McCallister.

Drills simulating a gunman in school have become standard practice in Colorado's schools.

For them, the shooting may bring up feelings of fear or hopelessness.

Remind them these drills are meant to keep them safe.

Help them learn techniques that decrease stress.

Positive self-talk, exercise, music, talking to a friend or playing with a pet may help, according McCallister.

Signs your child may need the help of a trained professional:

Worrying about dying at an early age

Losing interest in activities

Physical symptoms such as headaches and stomachaches

Sudden and extreme emotional reactions

Having problems falling or staying asleep

Showing irritability or angry outbursts

Having problems concentrating

Acting younger than their age (for example, clingy or whiny behavior, thumbsucking)

Showing increased alertness to the environment

Repeating behavior that reminds them of the trauma

For more information on helping your child cope with the post traumatic stress disorder visit The American Academy of Child and Adolescent Psychiatry or the U.S. Dept. of Veterans Affairs.