There have been at least 110 mass shootings since January 2009, roughly one to two shootings each month for the past six years. Women were more likely to be victims of the tragedies.

The statistics are part of a new report released by Everytown for Gun Safety, as Aurora, Colo., marks two years since the movie-theater massacre there. Just this month, a gunman opened fire in a suburb of Houston, Texas, killing two parents and four of their children.

Women comprised 14% of total gun homicide victims and 51% of mass shooting casualties, according to the study. Previous research recently published by Everytown showed that American women were 11 times more likely to be murdered with a gun than women in any other developed country.

The group, backed by former New York Mayor Michael Bloomberg, used FBI and media reports to analyze mass shootings in the country from January 2009 to July 2014. Researchers adopted the FBI’s definition of “mass shooting,” which categorizes each event as “a number of murders (four or more) occurring during the same incident.”

Among other discoveries, researchers disclosed that domestic violence influenced 57% of the incidents, when the shooter killed a current or former family member or partner. At least 42% of the mass shooters possessed their guns illegally, meaning they were felons, domestic abuse offenders, or otherwise restricted from possessing firearms, according to the analysis.

News stories often focus on a shooter’s mental health status. But the leaders of the study found that a minority — 11% — of perpetrators demonstrated signs of illness before the tragedy. The mass shooter’s age, on average, was 34.

Perpetrators who used high-capacity magazines, or assault weapons likely equipped with the ammunition, shot 156% more people, resulting in 63% more deaths, than in events without the involvement of such weapons. Magazines that can hold more than 10 rounds of ammunition are generally considered “large capacity”; some magazines can hold up to 100 rounds. High-capacity magazines are banned in eight states and the District of Columbia. In a controversial move, New Jersey Gov. Chris Christie earlier this month vetoed a gun control bill that would have banned magazines with more than 10 rounds of ammunition.

Less than one in six incidents — or 14% — took place in public spaces where firearms were not permitted, a figure that opposes the gun lobby’s claim that “gun-free zones” enable mass violence.

Everytown released the analysis just days before the two-year mark of when suspected gunman James Holmes killed 12 individuals and left scores wounded in a crowded Aurora movie theater during a midnight screening of “The Dark Knight Rises” on July 20, 2012.

Last year, Holmes pleaded not guilty by reason of insanity. He is currently being held without bail ahead of the trial, tentatively set for December to fulfill a doctor’s request for additional time to complete Holmes’ second sanity evaluation.

The national debate about gun rights is at a standstill as shooting rampages continue across the country. Congress failed to pass a bipartisan background checks bill last year, just months after the December 2012 massacre inside Sandy Hook Elementary School. Some states have passed new reform measures that tighten gun restrictions, while others have enacted laws that weaken regulations.

The Massachusetts House this month passed a sweeping gun bill that increases record-keeping efforts and attempts to curb gun trafficking. But earlier this week, legislators in the District of Columbia approved an amendment that would prevent officials in the territory from enforcing current gun laws supported by police and residents, including gun owners.

A study published in September found that the United States has more guns and gun deaths than any other developed country in the world. The two leading researchers who worked on the report discovered that the country has 89 guns per 100 people and 10 gun-related deaths per 100,000 people.

Two separate studies conducted by researchers at Texas A&M University and Georgia State University found that homicides increased under Stand Your Ground laws, which allows individuals to use deadly force to defend themselves without retreating from potentially harmful situations. The policy continues to be the subject of national debate, renewed after George Zimmerman’s acquittal last summer in the shooting death of Florida teenager Trayvon Martin.