The five stabbed and wounded at Hanukah celebration in New York City and two others shot dead in Houston, Texas past Saturday has accentuated the violent 2019 that the U.S. has experienced.

Even though the country has become synonymous with mass shootings over the last two decades with gun violence frequently happening every year in various forms, this year has been exceptionally violent. So much so that the database compiled by The Associated Press (AP), USA Today and Northeastern University shows that there were more mass killings in 2019 than any year dating back to at least the 1970s, punctuated by a chilling succession of deadly rampages during the summer.

The first one occurred 19 days into the new year when a man used an ax to kill four family members including his infant daughter. Five months later, 12 people were killed in a workplace shooting in Virginia. Twenty-two more died at a Walmart in El Paso in August.

In all, there were 41 mass killings, defined as when four or more people are killed excluding the perpetrator. Of those, 33 were mass shootings. More than 210 people were killed.

Most of the mass killings barely became national news, failing to resonate among the general public because they didn't spill into public places like massacres in El Paso and Odessa, Texas; Dayton, Ohio; Virginia Beach, Virginia; and Jersey City, New Jersey.

The majority of the killings involved people who knew each other – family disputes, drug or gang violence or people with beefs that directed their anger at co-workers or relatives.

In many cases, what set off the perpetrator remains a mystery.

That's the case with the very first mass killing of 2019, when a 42-year-old man took an ax and stabbed to death his mother, stepfather, girlfriend and 9-month-old daughter in Clackamas County, Oregon. Two others, a roommate and an 8-year-old girl managed to escape; the rampage ended when responding police fatally shot the killer.

The perpetrator had had occasional run-ins with police over the years, but what drove him to attack his family remains unknown. He had just gotten a job training mechanics at an auto dealership, and despite occasional arguments with his relatives, most said there was nothing out of the ordinary that raised significant red flags.

The incident in Oregon was one of 18 mass killings where family members were slain and one of six that didn't involve a gun.

The 41 mass killings were the most in a single year since the AP/USA Today and Northeastern database began tracking such events back to 2006, but other research going back to the 1970s shows no other year with as many mass slayings. The second-most killings in a year prior to 2019 were 38 in 2006.

James Densley, a criminologist and professor at Metropolitan State University in Minnesota, said the AP/USA Today/Northeastern database confirms and mirrors what his research into exclusively mass shootings has shown.

"What makes this even more exceptional is that mass killings are going up at a time when general homicides, overall homicides, are going down," Densley said. "As a percentage of homicides, these mass killings are also accounting for more deaths. "

He believes it's partially a byproduct of an "angry and frustrated time" that we are living in. Densley also said crime tends to go in waves with the 1970s and 1980s seeing several serial killers, the 1990s marked by school shootings and child abductions and the early 2000s dominated by concerns over terrorism.

"This seems to be the age of mass shootings," Densley said.

He and James Alan Fox, a criminologist and professor at Northeastern University, also expressed worries about the "contagion effect," the focus on mass killings fueling other mass killings.

"These are still rare events. Clearly the risk is low but the fear is high," Fox said. "What fuels contagion is fear."

The mass shootings this year include the three in August in Texas and Dayton that stirred fresh urgency, especially among Democratic presidential candidates, to restrict access to firearms.

While the large death tolls attracted much of the attention, the killings inflicted a mental and physical toll on dozens of others. The database does not have a complete count of victims who were wounded, but among the three mass shootings in August alone, more than 65 people were injured.