This still video image from a helicopter, courtesy of KABC-TV in Los Angeles shows the shooting scene in San Bernardino, Calif., on Dec. 2.  (KABC-TV via AFP)

Before the assailants opened fire at a holiday party in San Bernardino, Calif., on Dec. 3, killing 14 people and wounding 17 others, they left their 6-month-old baby with her grandmother.

Syed Rizwan Farook, 28, and Tashfeen Malik, 27, never returned. As a married couple, they formed an unusual profile among mass shooters in America. Women, acting alone or with an accomplice, are especially unlikely to go on a shooting spree.

Since 2003, 160 active shooters have killed 1,043 people and wounded 557 others in this country, according to an FBI analysis. Only six of the assailants were women.

Little research has examined women who commit mass shootings. A look at recent cases reveals one commonality: The attackers mostly targeted their places of work and education. Some sought revenge.  Others appeared to be suffering from mental illness.

In an effort to understand and stop gun violence, the FBI compiled information on every active shooter in the country from 2003 to 2013. The women here all used handguns:

  • On April 23, 2001, Cathline Repunte, a school bus driver, 36, shot four people and killed one at the Laidlaw Transit Services maintenance yard in San Jose, Calif. Deputy District Attorney Lane J. Liroff called Repunte "a frustrated and angry woman," but she never explained what prompted her to shoot her co-workers.
  • On Jan. 30, 2006, Jennifer San Marco, 44, fatally shot six people in her former workplace, the Santa Barbara U.S. Postal Processing and Distribution Center in Goleta, Calif. As police rushed to the scene, she committed suicide before explaining her actions. Prior to the attack, a manager at a mental health clinic reported seeing Marco talking to herself. "Nobody knew where she came from or what she was doing here," wrote a New York Times reporter. "People just knew there was something wrong."
  • On Feb. 8, 2008, Latina Williams, 23, fired six rounds into a second-floor classroom at Louisiana Technical College in Baton Rouge, La. Two people died in the gunfire. Williams didn't explain her reasoning before reloading the gun and killing herself. Police at the time said she'd exhibited signs of paranoia. Williams's mothers told authorities she had no idea what drove her daughter to kill. 
  • On Feb. 12, 2010, Amy Bishop Anderson, 44, a neurobiologist, sat in a biology department meeting in the Shelby Center at the University of Alabama in Huntsville, Ala., for 30 minutes before she stood up and shot six people. Three died and three were wounded. Anderson, who surrendered to police, had a clear motive: She'd been denied tenure
  • On March 30, 2010,  Arunya Rouch, 41, unleashed round after round into the parking lot of her former employer, a Publix Supermarket in Tarpon Springs, Fla., killing one person. She kept shooting as she walked into the store, passing customers on the way to her former boss’s office. Police arrived before she reached her destination, wounding Rouch in an exchange of gunfire. They learned that she'd just been fired
  • On Sept. 9, 2010, Yvonne Hiller, 43, was suspended from her job at the Kraft Foods Factory in Philadelphia  and escorted out of the building after getting into a fight with her co-workers. She quickly returned to shoot the people she felt had disrespected her, killing two and wounding one. Hiller also fired at responding officers, who quickly apprehended her. "She believed they were spraying chemicals at her, saying things behind her back," Philadelphia Homicide Capt. James Clark said.

Even more unusual than a woman on a shooting spree is a couple. Of the 160 active shooters in the FBI report, only two acted with anyone else.

Hussam Ayloush, executive director of the Council for American-Islamic Relations in Los Angeles, told The Washington Post that Malik, who was born in Pakistan, had been married to Farook for two years.  She left no web trace — no Facebook, no Twitter — but a baby registry: Pampers diapers, Johnson’s head-to-toe baby wash, a convertible car seat.

Before Malik, Amanda Miller was the last woman to fire into crowds with a romantic partner on U.S. soil. On June 8, 2014, Miller and her husband, Jared, both anti-government extremists, killed two police officers in a Las Vegas restaurant and then a man who tried to stop them in a Walmart. After police killed Jerad, Amanda committed suicide. Authorities said the couple identified with “militia and white supremacists" and believed law enforcement was the “oppressor."

When it comes to motive differences between the genders, science offers sparse insight.

Male and female brains aren’t inherently different, studies show. Social conditioning, however, may draw more men to violence. Wrote Steven Pinker, a psychologist, cognitive scientist and author of "The Better Angels of Our Nature": “Though the exact ratios vary, in every society, it is the males more than the females who play-fight, bully, fight for real, kill for real, rape, start wars and fight in wars.”

While insight into female mass shooters is limited, a study of female serial killers opens a window into what could be gender-specific motives: Women, the researchers found, are more likely to kill someone they know — or on behalf of someone they love. The weapon of choice is usually poison. Perhaps they’ve been personally wronged. Perhaps they’re after money. These killers often appear to be ordinary, non-threatening members of society until they strike.

Men are more likely to kill in response to a status threat or humiliation, another report suggests. An analysis of 263 cases in 29 countries of male-perpetrated mass murder found 86.9 percent of the crimes sprang from job loss, economic hardship or being bullied, according to researchers who studied the reported motives. 

Malik’s husband worked for the county, police said, and sprayed gunfire at his employer’s event.

Then there's the gender gap in weapon supply. Men are three times more likely than women to own guns, according to Gallup polls from 2007 to 2012.

More from Wonkblog:

The San Bernardino shooting is the second mass shooting today and the 355th this year

San Bernardino shooting: 11 essential facts about guns and mass shootings in America

How ‘active shooter’ became a depressing part of our vocabulary