Violence and mass shootings often surge in the summer months, especially around the Fourth of July, historically one of the deadliest days of the year.

A flurry of shootings around the holiday a year ago left more than a dozen people dead and over 60 wounded. Just two years ago, another mass shooting at a Fourth of July parade left seven people dead near Chicago. The mother of a 10-year-old boy left paralyzed by the attack said Wednesday that her family won’t go to this year’s parade, which is returning for the first time since the shooting.

“I don’t know if we’ll ever be able to attend the parade again,” said Keely Roberts, who also was wounded.

The Gun Violence Archive, which tracks mass shootings involving four or more people regardless of whether they died, shows June, July, and August have had the highest total number of mass shootings over the past decade. The lowest totals were from December through March.

Independence Day topped the list with 58 mass shootings over the last 10 years—closely followed by July 5, according to the archive.

“It’s the gathering, the free time, the drinking,” said James Alan Fox, a criminologist and professor at Northeastern University, who oversees a mass killings maintained by the Associated Press and USA Today in partnership with the university.

In the first half of this year, there were 19 mass killings—14 of them shootings—with at least four dead in the U.S., according to the database. In 2023, the nation recorded the highest number of mass shootings—39—since the tracking began.

Researchers point to a combination of factors that historically have caused the summer months to see an increase in violence and shootings.

School’s out for the summer

Mass killings are far more likely to happen at a home and most often the victims are related to the shooter or are a close acquaintance.

When school is out, families are spending more time together, children are often home all day and there’s a greater likelihood of more victims when everyone is under one roof, said Jesenia Pizarro, a criminology professor at Arizona State University.

Teenagers also have more idle time on their hands. “It’s like the opportunity shifts in the summer,” she said.

After two mass shootings hours apart in Dayton, Ohio, left a total of three people dead and eight injured in late June, police said one of the shootings took place at a vacant house where hundreds of teens and young adults had gathered.

“It could have been a lot worse,” said Eric Henderson, the city’s assistant chief, who pointed out it was the third big party since mid-June where trouble erupted after young people took over a vacant house.

More social events, more drinking

Family reunions, block parties and festivals in the summertime all bring more people together—and create more opportunities for trouble, more so when there’s drinking involved.

“It doesn’t mean that those kinds of things aren’t around in March or in January. They’re just around at a lower extent than they are in the summertime,” said University of Miami criminologist Alex Piquero. “We do know that just about every summer there’s an uptick in violence. So I fully anticipate that happening this summer. I fully anticipate it happening next summer and the summer after that.”

The likelihood of being a victim of a mass shooting is still extremely low, but it does mean there’s the potential for more victims if something happens at a crowded event.

During the first weekend of this summer, there were several shootings where multiple people were killed or wounded at large gatherings, including in Montgomery, Alabama, where gunfire erupted during an unsanctioned street party with more than 1,000 people. Police said nine people were shot and that investigators found more than 350 spent shell casings.

Tempers rise with hot temperatures

Several warm weather and hotter than normal temperatures with rising tempers—and not just in the summer. They also link the increased temps with more violent crimes, although other factors often come into play.

Former New York City police officer Jillian Snider, now a lecturer at the John Jay College of Criminal Justice, said she saw this firsthand in neighborhoods where a lack of air conditioning pushed people out onto their stoops or into parks on sweltering days.

“It makes people a little angry because there’s nowhere to cool down and tensions rise,” she said. “You have no escape from that, you’re just more upset.”

—Associated Press journalists Sharon Johnson in Atlanta and Sophia Tareen in Chicago contributed.