The Secret Service National Threat Assessment Center examined all 28 mass in 2017. Every act of violence was carried out by a man who “experienced at least one significant stressor within the last five years.”

The past year was rife with mass shootings and attacks— each incident perpetrated by men with a lot more in common than just their gender.

The Secret Service National Threat Assessment Center, in a study released Thursday, examined all 28 mass attacks between January and December 2017. At least three people were injured in each incident.

Every act of violence was carried out by a man who “experienced at least one significant stressor within the last five years,” according to the report.

Such stressors include family and romantic relationship problems, personal issues, difficult school and work environment, and previous contact with law enforcement that did not result in an arrest or charges.

About half of the attackers, around 54 percent, had a history of drug or substances abuse, and 20 of them—about 71 percent—had histories of criminal charges beyond minor traffic violations.

Nearly two-thirds of suspects experienced some sort of mental symptoms ahead of their attacks, though only 25 percent had been hospitalized or prescribed medication.

“The most common symptoms observed were related to psychosis”—including paranoia, hallucinations and delusions—“and suicidal thoughts,” according to the report.

Stephen Paddock, a reclusive gambler, staked out several outdoor concerts before raining bullets down on a Las Vegas country music festival from his 32nd floor hotel room in October. While his motive remains unclear, his family has a history of mental health issues and he lost a significant amount of wealth in the years leading up to his brutal attack.

Devin Patrick Kelley killed 26 people and wounded more than 20 during his November attack at First Baptist Church in Sutherland Springs, Texas. Previously a member of the U.S. Air Force, Kelley in 2012 was court-martialed for an assault on his wife and stepson. The same year, he escaped from a mental health clinic in New Mexico.

Both of these incidents should have prevented him from purchasing a firearm, but due to a reporting error he was able to obtain the weapon he used in the shooting.

Kelley killed himself moments after he opened fire into the Texas church. It’s one of the many instances of mass violence President Donald Trump has blamed on mental health. Trump has also taken aim at the Islamic State terror group, or ISIS, following events of mass violence.

“We must not allow ISIS to return, or enter our country after defeating them in the Middle East and elsewhere. Enough!” he tweeted after Sayfullo Sapov mowed down pedestrians on a Manhattan bike path.

Of the 28 men who committed acts of mass violence in 2017, seven “subscribed to a particular belief system,” according to the report. Only two of them were “self-radicalized followers of ISIS.”

Most of the incidents in 2017 were carried out using a firearm—82 percent of attackers used a gun, 11 percent used a vehicle and about 7 percent used knives, according to the report.

Of the 23 who used firearms, at least 10 possessed those weapons illegally. Two of them were minors and the others were either felons or had some other factor that should have prohibited them from owning a gun.

In the wake of the Parkland school shooting in February and October’s Las Vegas massacre—the deadliest mass public shooting in modern U.S. history—political leaders have called for a focus on mental health reform to curb similar attacks in the future.

Survivors of the Marjory Stoneman Douglas High School attack, on the other hand, have called for extensive background checks and gun reform following the Valentine’s day shooting.

While only 14 percent of the violent attacks have unfolded at schools, politicians have also recently suggested arming teachers so to protect young students. This stems from the fact that most attacks occur over the span of just minutes.

Half of the incidents unfolded in less than five minutes. Most them, 32 percent ended with law enforcement taking the attacker into custody while 25 percent are apprehended at a different location. Another 29 percent committed suicide as part of the incident and the remaining 14 percent were killed by law enforcement.