Here's what people who carried out 28 mass attacks in the US have in common: Secret Service

Seventy-nine percent of those who conducted a mass attack in the U.S. last year had previously engaged in threatening or concerning communications, with one-third threatening the specific target in some way prior to the attack, according to a new Secret Service report.

The report on 28 mass attacks in the U.S. in 2017, defined as one that harms three or more people, analyzes what drives someone to launch such an assault — and what others can look out for to help prevent them.

The attacks reviewed included high-profile incidents like the massacre at a Las Vegas country music festival on Oct. 1, which killed 58 people and injured hundreds more; the June 14 ambush on Republican lawmakers practicing baseball in Alexandria, Va.; and the Jan. 6 attack inside Fort Lauderdale-Hollywood International Airport, which left five dead and several others injured.

However, most of the deadly incidents reviewed never received the same type of national news coverage as those attacks.

Nearly half of the attacks were motivated by either a workplace grievance or ideology, including racism, according to the report.

All 28 mass attacks had one common thread - all were committed by males, ranging in age from 15 to 66, and 64 percent experienced mental health symptoms sometime prior to their attacks, according to the Secret Service.

A few months before 26-year-old Esteban Santiago opened fire last year in the baggage claim area of the airport in Ft. Lauderdale, Fla., Santiago had been sent for a mental health evaluation in Anchorage, Alaska. He had appeared at the FBI’s field office there speaking incoherently and claiming that the government was forcing him to watch videos produced by ISIS, according to ABC News reporting at the time.

He was released from an Alaska hospital on Nov. 14, 2016, without any psychiatric drugs; he was given anti-anxiety medication and melatonin, an herbal sleep aid.

Nevertheless, the Secret Service study found that 75 percent of perpetrators last year engaged in concerning but non-threatening communications, such as making overly angry statements, racist comments, or references to past attackers, according to the report.

The agency suggested that certain triggers could play a role in mass attacks, noting that all of the incidents last year were tied to at least one significant stressor, such as a romantic issue or an illness. In fact, 57 percent of those who carried out attacks last year experienced "stressors" related to financial instability in the five years before their attack, the Secret Service report said.

Seventy-one percent had histories of criminal charges beyond minor traffic violations. And a quarter of them were perpetrated by someone who previously faced charged related to domestic violence.

“The findings of the report support existing best practices the Secret Service has established in threat assessment and highlight the importance of a comprehensive investigation into an individual's background in order to assess potential risk,” the Secret Service said in a statement.

Eighty-two percent of the attacks were carried out with a firearm, and nearly half of those guns were obtained illegally. Knives and vehicles were also used to harm people in mass attacks last year.

"Our behavioral research on incidents of targeted violence has shaped how we conduct threat assessments as an agency,” the Assistant Director of the Secret Service’s Office of Strategic Intelligence and Information, Frederick Sellers, said in a statement. “We use multiple sources to gather and analyze information to assess concerning behaviors and identify mitigation strategies in support of our protective mission.”