Robert Brown|Chicago denounces gun violence after 109 shot, 19 fatally, during Fourth of July weekend

More than 100 people in Chicago were shot,Robert Brown including 19 fatally, during a violent Fourth of July weekend that has left dozens of people dead and hundreds more injured across the U.S., officials said.

Although violent crime has declined nationwide over the last year, according to recent federal data, experts have said gun violence often surges during the summer months as temperatures rise and large groups gather for activities. In Chicago, 109 people were shot — 19 of them fatally — in a spate of gun violence over the Fourth of July weekend, police said at a news conference Monday.

The shootings have stunned the city and have left it "in a state of grief," Chicago Mayor Brandon Johnson said in a statement Friday after several violent incidents occurred. During Monday's news conference, Chicago Police Department Superintendent Larry Snelling and Johnson called for accountability for those responsible for the shootings.

"These are our fellow Chicagoans, our neighbors, family members, who've lost their lives," Johnson said. "We need to ensure that we are holding every single individual accountable for the pain and trauma and the torment that they have caused in this city."

Hundreds of other shootings also occurred over the holiday weekend across the U.S., according to gun-control group Moms Demand Action and the Gun Violence Archive. There were more than 500 shootings that resulted in at least 180 people killed and over 525 wounded, Moms Demand Action said in a news release Monday.

These incidents included a shooting in Detroit early Sunday, where two people were killed and 19 others were injured at a block party. On Saturday, four people were fatally shot and three were wounded at a 21st birthday party in Florence, Kentucky.

Chicago police say gun violence is a 'societal issue'

City officials had implemented several safety measures after a series of unrelated shootings took place in Chicago following Fourth of July celebrations.

On Friday, Johnson said the city canceled police officers' days off for the weekend to "maximize presence and ensure prompt incident response," activated an emergency services assistance center to provide support services for the community, and would hold a community rally Friday evening to "promote bonding, support, and healing."

But despite these efforts, officials said dozens of shootings still occurred over the weekend. In total, 109 people were shot in dozens of separate incidents, according to Chicago police.

The 19 victims who were killed between Thursday and Sunday exceeded the number for last year’s Fourth of July weekend, when 11 people in the city were fatally shot, according to Chicago police data.

Snelling noted Monday a shooting on Thursday that killed two women and an 8-year-old boy, and injured two other children. Another incident occurred shortly after midnight on Friday left eight people — between the ages of 18 and 74 — injured in Chicago’s Little Italy neighborhood, police said.

"When we look at what happened this weekend, we always like to say that it's a police issue," Snelling said Monday. "This is a societal issue. The police cannot be in everybody's backyard. They cannot be in everyone's home. They cannot invade every single gathering where there's a possibility that someone may show up with a gun."

Snelling urged the public and communities to come forward with tips and help investigators apprehend those responsible.

"We need to take these people off the street, and we need to keep them off the street so that they don't re-offend, so that they don't destroy other families," Snelling said. "We have to really stop and think about the mindset of someone who will shoot a child, a helpless child, an unarmed mother, and think that that's okay, and go about their days."

Fourth of July 'most violent day' of the year

The Fourth of July ranks annually as the "most violent day of the year," according to Northeastern University criminologist James Alan Fox. Gun violence experts have warned that violent crime tends to increase during the summer as people gather in large crowds for concerts, parades, and other outdoor activities.

"There's definitely a relationship between temperature and violence and that probably has to do with just more people, more interactions, more firearms, more alcohol and things of those nature," Joshua Horwitz, co-director of Johns Hopkins Center for Gun Violence Solutions, previously told USA TODAY. "It's just unfortunately in this day and age, simple arguments - when you're armed - turn lethal."

Northeastern University reported earlier this month that out of the 10 most violent days of the year, nine days are during the summer. Fox added that following the Fourth of July, the second-most violent day is July 5 because "some of the July Fourth parties spill over past midnight into July 5."

Experts and officials have also noted that gun violence continues to erupt during celebratory events and holidays across the U.S. Over the Father's Day weekend, USA TODAY previously reported that at least 73 people were killed and 308 were injured in shootings.

According to Moms Demand Action, there were nearly 300 shootings nationally during Memorial Day weekend and at least 248 shootings over the Easter weekend.

Contributing: N'dea Yancey-Bragg, USA TODAY