Updated

Two suspected gunmen and two others have been charged in last week's shooting of 13 people including a 3-year-old boy in a crowded Chicago park, police said Tuesday.

Police Superintendent Garry McCarthy said the gunmen opened fire on the group in retaliation for an earlier shooting in which one of them was slightly wounded. He said there was no specific target, but that the shooters went to the park because it was rival gang territory.

"There's a super-heated group of gunmen who were victims and offenders," McCarthy said. "This individual was a victim of gun violence and then became an offender."

Police said all four suspects were charged with attempted murder and aggravated battery in Thursday's attack in Cornell Square Park on the southwest side.

Investigators said the shooters where Tabari Young, 22, and 21-year-old Bryon Champ, who each opened fire on the crowded park as people played a nighttime basketball game on a warm late-summer evening.

Young, who was identified as the principal gunman, used a military-style weapon, while Champ used another gun, McCarthy said.

Twenty-two-year-old Brad Jett, a suspected lookout, and 20-year-old Kewane Gatewood, who is accused of supplying a gun, were also charged, McCarthy said.

Young and Jett were charged Tuesday while Champ and Gatewood were charged Monday.

The shooting injured two teenagers and 3-year-old Deonta Howard, who is recovering from surgery after being shot near his ear.

Authorities say Young has more than a dozen previous arrests and have described Champ as a felon and a documented gang member. They say the attack was in retaliation for an unreported shooting that took place earlier in the day in which Champ suffered a graze wound.

Champ, who was convicted in July 2012 on charges of unlawful use of a weapon by a felon, was previously sentenced to Cook County Jail's boot camp.

McCarthy and Mayor Rahm Emanuel cited the charges and Champ's record to reiterate their demands for tougher gun laws in Illinois.

"We need a three-year minimum penalty for illegally carrying a gun on our streets," Emanuel said. "One of the shooters should have been behind bars rather than in Cornell Park on Thursday night."

McCarthy said the investigation continues, and more people could be charged.

Police had recorded 306 murders and 1,402 shooting incidents this year in Chicago as of Sept. 15, down 20 and 22 percent, respectively, from the same period in 2012. Police say overall violent crime is down about 15 percent, according to new figures released Monday.