Adrees Latif/Reuters Mourners hold a picture of their deceased friend, Xavier Emanuel Serrano Rosado, during a vigil one day after a mass shooting at the gay Orlando nightclub Pulse. The shooter, Omar Mateen, had a history of domestic violence.

Long before 29-year-old Omar Mateen opened fire in a popular gay nightclub in Orlando, Florida, and committed the deadliest mass shooting in modern U.S. history, he perpetrated a different type of terror -- one he kept hidden inside the home.

He beat his wife.

Sitora Yusifiy, who was married to Mateen for four months, has described him as a cruel and physically violent man. She said that he beat her over things like unfinished laundry, confiscated her paychecks and banned her from speaking to her family -- acts that left her feeling isolated, vulnerable and afraid.

"He would often get into fights and arguments with his parents but because I was the only one in his life most of the violence was towards me at that time," she told reporters from her home in Colorado, where she eventually moved after divorcing Mateen in 2009.

"My family literally rescued me," she said. "They had to pull me out of his arms and buy an emergency flight. I left all my belongings."

Seven years later, Mateen's violence would be directed outward, toward Orlando's vibrant gay community. On early Sunday morning, he stormed the nightclub Pulse, slaughtering 49 people and injuring another 53 before police killed him after an hourslong standoff.

While Yusifiy says she never saw Mateen support terrorism, public discussion has focused on his potential terrorist connections after reports that he called 911 and pledged allegiance to the so-called Islamic State. Did he actually have ties to ISIS? Did he harbor anti-American sentiments? Did he in fact cheer when he heard about the Sept. 11 attacks? We don't know all the answers yet, but by singlehandedly focusing on this line of questioning, we risk overlooking more critical indicators.

To understand if authorities missed clues that Mateen was capable of carrying out a massacre, we must look to his history of domestic violence -- and that of so many before him who have committed mass killings.

Time after time, men (and yes, it is mostly men) who commit horrific acts of violence against the public have a history of domestic violence against the women closest to them.

Before Robert Dear shot to death three strangers in a Planned Parenthood in Colorado Springs last fall, he allegedly beat his wives, targeted other women he came into contact with, was charged with rape and arrested under a "Peeping Tom" law.

Before Tamerlan Tsarnaev planted bombs at the Boston Marathon with his brother in 2013, killing three people and injuring more than 260 others, he was arrested for beating his girlfriend.

In February, Cedric Ford stormed through multiple Kansas townships with an assault rifle and a pistol, killing three and injuring 14. Ninety minutes earlier, he'd been served him with a restraining order stemming from a domestic violence complaint filed by his ex-girlfriend.

In 2014, when gunman Man Haron Monis seized hostages in a cafe in Sydney, Australia, he was out on bail after being charged as an accessory to the murder of his ex-wife.

And so on. The examples are too numerous to list.

As Shannon Frattaroli, an associate professor at the Johns Hopkins Bloomberg School of Public Health, told Vox, "We know that violence is the best predictor of future violence."

In fact, according to one analysis of criminal offenders in Washington State, a felony domestic violence conviction may actually be the single greatest predictor of future violent crime among men.

"Men who commit violence rehearse and perfect it against their families first," wrote activists Pamela Shifman and Salamishah Tillet in a New York Times op-ed last year. "Women and children are target practice, and the home is the training ground for these men’s later actions."

The Huffington Post analyzed five years of data on mass shootings (commonly defined one in which at least four people are killed) compiled by the gun violence prevention organization Everytown For Gun Safety. We found that in 57 percent of mass shootings, the shooter targeted either a family member or an intimate partner.

It's unclear if police were ever notified of Mateen's alleged violence against his ex-wife, and he does not appear to have any convictions for domestic violence, which would have prohibited him from owning guns under federal law.

But even if Yusifiy did not report his abuse to authorities, as many domestic violence victims do not, she was not the only person who bore witness to his misogynistic tendencies and erratic behavior.

One of Mateen's co-workers, Daniel Gilroy, told The Los Angeles Times that he "complained multiple times that [Mateen] was dangerous" and that he showed anger toward black people, gay people, Jewish people and women.

"He did not like black people at all. That was mentioned once or twice, but more so was women," Gilroy told NBC News. "He did not like women at all. He did like women in a sexual way, but he did not respect them."

Of course, not all domestic abusers will commit horrific acts of violence against the public. But when men abuse their partners, they are broadcasting that they are violent, dangerous and potentially able to turn that violence against others. It's a warning sign that should be taken seriously.

We know that domestic violence is quite common in the U.S., where it affects 1 in 4 women and every single day claims three women's lives. Now, it's critical that we begin to recognize it as a key warning sign of a person's potential to commit violence against others. It is a serious issue worth tackling as a public health epidemic -- not just something private that occurs behind closed doors.

"Possible ties to terrorist groups seem to be more interesting and frightening to the public than a history of domestic violence, which sadly has become part of the everyday fabric of our society," said Kim Gandy, president of the National Network To End Domestic Violence.

"Yet by overlooking the fact that many domestic violence perpetrators kill multiple victims – including but not limited to mass shootings – we are missing an opportunity to prevent some of this deadly violence, through better enforcement of laws prohibiting domestic abusers’ access to firearms, and by holding abusers accountable for their behavior when it is initially identified."

Need help? In the U.S., call 1-800-799-SAFE (7233) for the National Domestic Violence Hotline.

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