A spate of weekend mass shootings and violence across the U.S. killed at least six people, including a Pennsylvania state trooper, and left dozens injured.

The shootings follow a surge in homicides and other violence over the past several years that experts say accelerated during the coronavirus pandemic. They happened in suburban Chicago, Washington state, central Pennsylvania, St. Louis, Southern California and Baltimore.

“There’s no question there’s been a spike in violence,” said Daniel Nagin, a professor of public policy and statistics at Carnegie Mellon University. “Some of these cases seem to be just disputes, often among adolescents, and those disputes are played out with firearms, not with fists.”

Researchers disagree over the cause of the increase. Theories include the possibility that violence is driven by the prevalence of guns in America, or by less aggressive police tactics or a decline in prosecutions for misdemeanor weapon offenses, Nagin said.

As of Sunday evening, none of the weekend events fit the definition of a mass killing, because fewer than four people died at each location. The figure does not include the shooter. However, the number of injured in most of the cases matches the widely accepted definition for mass shootings.

Here's a look at the shootings this weekend:

WILLOWBROOK, ILLINOIS

At least 23 people were shot, one fatally, early Sunday in a suburban Chicago parking lot where hundreds of people had gathered to celebrate Juneteenth, authorities said.

The DuPage County sheriff’s office described a “peaceful gathering” that suddenly turned violent as a number of people fired multiple shots into the crowd in Willowbrook, Illinois, about 20 miles (32 kilometers) southwest of Chicago.

A motive for the attack wasn’t immediately known. Sheriff’s spokesman Robert Carroll said authorities were interviewing “persons of interest” in the shooting, the Daily Herald reported.

A witness, Markeshia Avery, said the celebration was meant to mark Juneteenth, Monday's federal holiday commemorating the day in 1865 when enslaved people in Galveston, Texas, learned they had been freed, two years after the Emancipation Proclamation.

“We just started hearing shooting, so we dropped down until they stopped,” Avery told WLS-TV.

The White House issued a statement calling the violence a tragedy and saying the president was thinking of those killed and injured. Illinois Gov. JB Pritzker said in a statement that he was monitoring the investigation.

“Gathering for a holiday gathering should be a joyful occasion, not a time where gunfire erupts and families are forced to run for safety,” Pritzker said.

WASHINGTON STATE

Two people were killed and two others were injured when a shooter began firing “randomly” into a crowd at a Washington state campground where many people were staying to attend a nearby music festival on Saturday night, police said.

The suspect was shot in a confrontation with law enforcement officers and taken into custody, several hundred yards from the Beyond Wonderland electronic dance music festival.

A public alert advised people of an active shooter in the area and advised them to “run, hide or fight."

The festival carried on until early Sunday morning, Grant County Sheriff’s Office spokesman Kyle Foreman said. Organizers then posted a tweet saying Sunday’s concert was canceled.

CENTRAL PENNSYLVANIA

One state trooper was killed and a second critically wounded just hours apart in central Pennsylvania on Saturday after a gunman attacked a state police barracks.

The suspect drove his truck into the parking lot of the Lewistown barracks about 11 a.m. Saturday and opened fire with a large-caliber rifle on marked patrol cars before fleeing, authorities said Sunday.

Lt. James Wagner, 45, was critically wounded when he was shot after encountering the suspect several miles away in Mifflintown. Later, Trooper Jacques Rougeau Jr., 29, was ambushed and killed by a gunshot through the windshield of his patrol car as he drove down a road in nearby Walker Township, authorities said.

The suspect was shot and killed after a fierce gunbattle, said Lt. Col. George Bivens, who went up in a helicopter to coordinate the search for the 38-year-old suspect.

“What I witnessed ... was one of the most intense, unbelievable gunfights I have ever witnessed,” Bivens said, lauding troopers for launching an aggressive search despite facing a weapon that “would defeat any of the body armor that they had available to them.”

A motive was not immediately known.

ST. LOUIS

An early Sunday shooting in a downtown St. Louis office building killed a 17-year-old and wounded nine other teenagers, the city’s police commissioner said.

St. Louis Metropolitan Police Commissioner Robert Tracy identified the victim who was killed as 17-year-old Makao Moore. A spokesman said a minor who had a handgun was in police custody as a person of interest.

Teenagers were having a party in an office space when the shooting broke out around 1 a.m. Sunday.

The victims ranged from 15 to 19 years old and had injuries including multiple gunshot wounds. A 17-year-old girl was trampled as she fled, seriously injuring her spine, Tracy said.

Shell casings from AR-style rifles and other firearms were scattered on the ground.

SOUTHERN CALIFORNIA

A shooting at a pool party at a Southern California home left eight people wounded, authorities said Saturday.

Authorities were dispatched shortly after midnight in Carson, California, south of Los Angeles, KABC-TV reported.

The victims range in age from 16 to 24, the Los Angeles County Sheriff’s Department said in a statement. They were taken to hospitals and two were listed in critical condition, the statement said.

Authorities said they found another 16-year-old boy with a gunshot wound when they responded to a call about a vehicle that crashed into a wall nearby.

BALTIMORE

Six people were injured in a Friday night shooting in Baltimore. All were expected to survive.

Officers heard gunshots in the north of the city just before 9 p.m. and found three men with numerous gunshot wounds. Medics took them to area hospitals for treatment.

Police later learned of three additional victims who walked into area hospitals with non-life-threatening gunshot wounds.

The wounded ranged in age from 17 to 26, Baltimore Police Department spokesperson Lindsey Eldridge said.