At least six people including a Pennsylvania state trooper were killed and dozens injured in a string of weekend violence and mass shootings across the U.S.

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

“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. However, the number of injured in most of the cases does match the widely accepted definition for mass shootings.

Here's a look at the shootings this weekend:

WILLOWBROOK, ILLINOIS

One person was killed and at least 22 wounded in a shooting early Sunday during a party in a suburban Chicago parking lot.

The DuPage County sheriff’s office described a “peaceful gathering” to celebrate Juneteenth that suddenly turned violent in Willowbrook, about 20 miles southwest of Chicago.

A motive for the attack wasn’t immediately known, and as of early afternoon, no one had been arrested.

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,”w Avery told WLS-TV.

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 people stayed 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.

A man engaged troopers at about 12:45 p.m. Saturday near the Lewistown barracks and shot one trooper who was taken to a hospital with serious injuries, police in Juniata County said.

The suspect fled, but was found about two hours later several miles away in Walker Township. A gun battle erupted, killing the shooter and mortally wounding a second trooper, police said.

Neither troopers' names, nor the suspect's were immediately released.

Gov. Josh Shapiro said in a social media post that the wounded officer was in critical but stable condition.

The governor said he and first lady Lori Shapiro “send our love to the family of the trooper killed today. May his memory be a blessing.”

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.

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

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.