CHICAGO — At least 81 people have been shot in the nation’s third-largest city since Monday, a troubling uptick of violence for a metropolis that has seen some recent success in reducing shooting incidents.

The surge in violence, which includes six people who have been fatally shot, comes as Chicago Police Department officials have expressed optimism in recent months that gun violence was on the downward trend in a city that tallied more than 1,400 homicides in 2016 and 2017 combined.

Chicago recorded a 22.3% reduction in murders and a 26.5% decline in shooting incidents for the first four months of 2018 compared with the same period in 2017, according to police department data. April also marked the 14th consecutive month in which Chicago recorded a decline in gun violence, according to police department spokesman Anthony Guglielmi.

But as the weather has warmed  — Chicago endured the fourth-coldest April on record this year but saw temperatures rise this week — the city has seen a spasm of violence.

At least 39 people were shot between Friday and early Sunday, according to police.

Police and federal authorities on Sunday continued the hunt for an assailant or assailants who critically wounded an ATF agent a day earlier on the city’s Southwest Side. Police said the agent, who was shot in the face but is expected to survive, was part of a newly-created strike force – a partnership between federal and local law enforcement – aimed at stemming gun violence in the city.

Police officials said several of the incidents, including a drive-by shooting late Friday in which a 41-year-old man and 17-year-old boy were wounded, appear to be gang-related. In two separate shootings Saturday afternoon on the West Side, a 26-year-old man and a 25-year-old man who police said are documented gang members were shot. The 25-year-old who was shot multiple times as he rode a bike was rushed to a nearby hospital and listed in critical condition, police said.

In the latest fatal shooting, a 46-year-old man was killed on the city’s South Side early Sunday after being shot in the chest.

At least a few of the wounded were children.

Around 9:20 p.m. Saturday, a 12-year-old boy was critically injured after he was shot in the stomach. Police said the boy was wounded by a gunman who fired into a crowd on a sidewalk on the city’s West Side. Police were searching for the assailant, who was arguing with a woman before allegedly firing the weapon.

Early Sunday morning, two men, ages 22 and 24, were wounded as they walked out of a building on the same block where the 12-year-old boy was shot hours earlier. It was not immediately clear if the two incidents were related.

In another incident involving a child, a girl, 4, who was shot in the shoulder Tuesday evening as she was with her parents on the porch of their home on the city’s South Side. Police say someone in a dark-colored sedan pulled up near the house before opening fire.

Some of the victims appeared to simply have terrible luck. In an incident Wednesday evening, a 41-year-old man was struck in his thigh and leg as he lay in bed in his apartment on the city’s West Side. Police said the bullet was fired by somebody in an adjacent apartment. That suspect fled before police arrived at the scene.

Another victim, a 15-year-old boy, was struck in the head by a bullet Wednesday afternoon on his way home from school on a city bus on the city’s Southwest Side. The boy, who is listed in good condition, traveled several miles from his home to attend one of the city’s most academically selective high schools.

“It infuriates me that we have a good kid doing what we all expect him to do, and he’s a victim of something like this,” Police Superintendent Eddie Johnson told reporters. “This is why we have to have common sense gun legislation in this country. Not just this city, this state, but in this country, to stop things like this from happening.”

Police say they don’t believe the teen was targeted but was struck by an errant bullet.

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Despite the uptick in violence this week, city officials say they are having better luck fighting gun violence as a result of investing millions in improved technology since late 2016.

Chicago has partnered with the University of Chicago Crime Lab to build data-driven support centers in 11 of the city’s districts — an area that covers about 100 square miles — that use hyper local video, shot spotter and other data to help better deploy officers in the city and more quickly respond to violent crime.