WASHINGTON — The FBI said Wednesday that a study of terror attacks in the United States over the past three decades revealed that lone offenders were seldom completely isolated and alone, further evidence that following the government's "see something, say something" advice can help prevent attacks.

While predicting lone offender terrorism is not possible, “it may be preventable with increased bystander education and awareness in recognizing concerning behavior and reporting it to authorities as soon as possible," the FBI said in releasing a report compiled by the Behavioral Threat Assessment Center of the bureau's Behavioral Analysis Unit — better known as the FBI’s profilers.

Most offenders had “family, peers, or online contacts who were in a position to notice troubling behavior," the report said. "More than half of those who observed concerning behaviors made some effort to intervene or voice their concerns."

The report examined 52 lone offender terrorist attacks in the U.S. from 1972 to 2015. Thirty-three of their acts of terrorism killed a total of 258 people and injured 982. But it found nothing to suggest a profile of a typical attacker, other than the fact that all were men.

Most were U.S. citizens, born here, single and had some college education. But more than half were not working and not in school when they carried out their attacks.

The majority of them "appeared to experience mental health stressors," though most were never diagnosed with any psychiatric disorder. Of the 13 who were, mood disorders such as depression and bipolar disorder were the most common diagnosis. And more than half of the offenders "demonstrated a pervasive distrust and suspiciousness of others or a belief that others were plotting to harm them."

The researchers found that anti-government violent extremism was the most common motivation during the period they studied, but that trend may be changing.

FBI Director Christopher Wray told Congress earlier this month that "a huge chunk" of the FBI's current domestic terrorism investigations involve "racially-motivated, violent, extremist-motivated terrorist attacks." Of those, Wray said, the majority "are fueled by some kind of white supremacy."