Mass shootings left three people dead in Nevada and killed six in Texas this week, the most recent attacks in a year that has seen an acceleration of mass killings in the United States.

Mass killings, defined as four or more people killed, not including the perpetrator, remain rare. But as the year comes to a close, the United States is approaching its record for the number of such incidents in a single year. So far in 2023, there 42 have been mass killings in the country, according to the USA TODAY/Northeastern University/Associated Press Mass Killings Database. In 2019, the year a gunman shot and killed 23 people in a racist attack at an El Paso, Texas, Walmart, the country experienced a record 46 mass killings.

"We could match it, we could beat it, but we'll be close regardless," said James Alan Fox, a Northeastern University professor who has studied mass killings for decades. "Whether it's a record or not, it's obviously quite disturbing that we are creeping towards that high mark."

Here's what to know about mass killings and mass shootings in the United States in 2023:

How many mass killings in 2023?

Of the more than 40 mass killings so far in 2023, only three did not involve a gun – including a knife attack in New York on Sunday during which a man allegedly killed four relatives, set a building on fire and stabbed two police officers before he was fatally shot.

Mass killings appear to be accelerating in the United States. There were at least 30 mass killings across the nation by July, more at that point in the year than any other since at least 2006, according to the USA TODAY/Northeastern University/Associated Press database.

The country typically experiences about six mass shootings in public places each year, according to Fox, who manages the database. There have been 10 such shootings in 2023 – the most the country has ever witnessed in a year, he said.

Those incidents include the shooting on the campus of the University of Nevada, Las Vegas, on Wednesday and the rampage in Austin and San Antonio on Tuesday, which marked the 50th mass killing incident in Texas in the last 15 years, according to the database. Those attacks come after other mass killings in Maine, Georgia, California, Kentucky, Alabama, Pennsylvania, Arizona and Tennessee gained national attention this year.

How many mass shootings in 2023?

This year, there have been at least 632 mass shootings across the nation, leaving at least 1,336 people dead and 2,625 injured, according to data from the Gun Violence Archive, a nonprofit publicly sourced database. The database defines a mass shooting as at least four people struck by gunfire, not including the shooter.

According to the archive, there are only seven states without a mass shooting in 2023: Alaska, Montana, Rhode Island, South Dakota, Vermont, West Virginia and Wyoming. The number of mass shootings has risen from a low of 272 in 2014 to a high of almost 690 in 2021, according to the archive.

What's causing the increase?

Fox said the prevalence of weapons, lax gun laws and high-capacity magazines have all affected the rate of mass shootings. In a statement Wednesday, President Joe Biden urged Congress to pass legislation to address the "epidemic of gun violence," including banning assault weapons and high-capacity magazines.

"We must do more to prevent more families, and more communities like Austin, San Antonio, and Las Vegas, from being ripped apart by gun violence," Biden said.

Fox also said several factors unrelated to guns have had an impact. He pointed out that Texas and California, the country's most populous states, have seen high numbers of mass killings.

"Since 2006, when the database began, we've had about a 12% increase in population, so part of the increase is driven by a larger number of people," he said. "But the pandemic and economic hardship and the political environment have all played a role."

Could this trend get worse?

Fox said the numbers of mass shootings and mass killings have increased in recent years, adding six of the nine mass shootings in the country's history with at least 20 fatalities have occurred since 2012. Nevada became the site of the nation's deadliest mass shooting in 2017, when a gunman killed 60 people at the Route 91 Harvest Festival, miles away from where Wednesday's shooting occurred.

But, he said though fear of mass shootings continues to rise, these incidents remain rare. And, ultimately, it is difficult to predict whether mass shootings and mass killings will increase.

"There's so many social and economic factors that underscore these cases, it's just hard," he said. "You can't predict just based on the trend."

Contributing: Bayliss Wagner, Austin American-Statesman; Grace Hauck, USA TODAY; The Associated Press