Looking for a motive or root cause of Thursday's shooting in Aberdeen, investigators said they interviewed family and friends of the shooter, Snochia Mosely, who took her own life. Moseley was a licensed gun owner who was diagnosed with a mental health disorder in 2016.

"Evidence that the shooter was suffering from a mental illness was identified. Friends and family indicated (that) over the last few weeks she had become extremely agitated and they were concerned for her well-being," said Sheriff Jeffrey Gahler, of the Harford County Sheriff's Office.

That is a red flag, according to experts, but they also said it is a mistake to consider mental health as the primary driver of violence.

"About 5 percent of homicides are committed by someone who has a serious mental illness," said Paul Nestadt, a psychiatrist at Hopkins Hospital.

Nestadt said there are all kinds of factors that can come into play, including access to guns.

"Even a professional can't really, reliably predict violence. It's difficult to predict," he said.

Nestadt spoke to 11 News about the available research on mass shootings.

"Many of these mass shooters, it's the suicidal act. That is the primary driver, and for whatever reason, they take people with them. Something going on in that final act, they commit violence against others. The primary goal is to die," Nestadt said.

For those in Harford County and other communities, how to prevent such violence is a growing concern.