Communities across the country are grappling with a rash of violence in the final days of spring that included at least four mass shootings in one weekend, and continued Monday with stabbings outside an Indiana strip mall.

On Saturday, a lone gunman opened fire at a recreational splash pad in suburban Detroit, injuring nine people including an 8-year-old who was shot in the head. In Texas, fatal gunfire broke out at a Juneteenth celebration, with two people pronounced dead.

Other shootings and stabbings, in locations from Massachusetts to Indiana, marred the days before spring turns to summer as sweltering heat takes hold in many regions.

Chris Dennison, a sociology professor at the University of Buffalo who researches crime, said the arrival of seasonable weather presents more opportunities for crime and violence, amid heightened concerns about mental health issues and destructive outcomes.

“More people are out, more people are traveling, days are longer, weather is better: People are just far more inclined to be out and about, and with that, the potential for crime is just invariable there as well,” Dennison said Monday.

The rash of violence is a counterpoint to FBI data pointing to a sharp drop in violent crime nationwide earlier this year.

Brian Higgins, a lecturer at John Jay College of Criminal Justice in New York, said summer heat, large social gatherings and alcohol consumption typically fuel a seasonal increase in violence.

“This is to be expected,” said Higgins, a former police chief for Bergen County, New Jersey. “Is it worse than other years? That remains to be seen.”