Criminologists who study mass killings call it nearly unprecedented for a married couple to carry out a massacre like the one that left 14 people dead in California.
Officials in San Bernardino believe that both 28-year-old Syed Farook and his 26-year-old wife, Tashfeen Malik, fired weapons in the gunfire that killed or wounded more than 30 people at an office holiday party. They were killed in a battle with police hours later.
"Given gender politics, I would assume some people will say this woman (Malik) must have been hoodwinked or pressured or coerced by her husband. And it's possible. But it's possible they were co-equal or it's possible he was pressured," said criminologist Adam Lankford of the University of Alabama, who said his research concluded that this is the first U.S. public mass shooting in recent history to involve a male-female team.
A forensic psychiatrist who also studies mass killings said there's not enough evidence yet to understand what might have prompted such a pact between spouses.
"This is very different and I don't think anybody understands it yet," said Dr. Douglas Mossman of the University of Cincinnati College of Medicine. "Typically with the way that shooters ... are described, they're alienated; it seems they don't have a great deal to look forward to; they're isolated and alone, or at least feel that way. That's not the typical situation for people who are married."
The list of deadly U.S. crime sprees perpetrated by spouses or romantic partners include:
— May 1934: Notorious bank robbers Clyde Barrow, 25, and Bonnie Parker, 23, die in a police ambush in Bienville Parish, Louisiana. From 1932 to 1934, the couple was part of an ad hoc gang of outlaws that robbed and shot its way from Texas to Minnesota in a crime spree involving stores, gas stations and a few banks. The tally of deaths attributed to them numbers about 12 police and civilians, plus various kidnappings
— January 1958: 14-year-old Caril Ann Fugate joined boyfriend Charles Starkweather during a Midwest crime spree that left her parents, sister and seven others dead. He was executed; she insisted she had merely rode along with him and was paroled after 17 years.
— July 1984: Former FBI "most-wanted" fugitive Alton Coleman, 28, and his girlfriend, 22-year-old Debra Brown, both of Waukegan, Ill., were arrested after a seven-state crime spree that left as many as eight people dead and included rapes, kidnappings and armed robberies. Coleman was executed; Brown remains on death row in Indiana.
— October 1996: Police arrested John Esposito, 24, of Berlin, New Jersey, and Alicia Woodward, 22, of Marlton, New Jersey, after a seven-state crime spree that left three elderly people bludgeoned to death. Esposito was sentenced to death while Woodward got a life sentence.
— June 2014: Amanda Miller, 22, and her 31-year-old husband, Jerad Miller, shot and killed two police officers in a pizza shop in Las Vegas and then went to a Wal-Mart where they announced plans to start a revolution. The couple died there in a gunfight with police, but not before Amanda Miller killed a good Samaritan who intervened.
Associated Press researcher Jennifer Farrar contributed to this report.