(CNN) -- Chicago's citywide crime wave didn't slow down during the holiday weekend.
City police investigated 27 shooting incidents this Christmas weekend, 12 of which were fatal, city Police Superintendent Eddie Johnson said during a Monday news briefing.
"The violence primarily occurred in areas with historical gang conflicts on the South and West sides of the City," Johnson said. "We now know that the majority of these shootings and homicides were targeted attacks by gangs against potential rival gang members and groups who were at holiday gatherings."
Chicago police said there have been 753 homicides and 3,495 shooting incidents in the city from January 1 to December 25. During the same time frame in 2015, there were 478 homicides and 2,393 shooting incidents.
"These were deliberate and planned shootings by one gang against another," Johnson said Monday. "They were targeted knowing fully well that individuals would be at the homes of family and friends celebrating the holidays. This was followed by several acts of retaliation."
In one Christmas night incident, a man walked out of an alley, opened fire on people partying on a porch in the East Chatham neighborhood and then ran away, CNN affiliate WLS reported. Two brothers 18 and 21 years old died and five people were wounded. No arrest has been reported.
Ninety percent of those killed had gang affiliation, criminal history and had been identified as potential offenders or victims of gun violence, said Johnson. Forty-five guns were seized over the weekend, he said.
Johnson called on Chicago's policymakers to enact stricter gun laws, specifically to increase sentences for repeat gun violence offenders.
Johnson said criminals feel empowered and emboldened by recent criticisms of police.
"When they feel the public will speak out for them and not the police officers, that's giving them the power to go out and do what they did," Johnson said.
August was the deadliest month, with 96 homicide reports, the Chicago Tribune said. In one weekend in August, there were eight gun-related homicides and 64 non-fatal shootings.