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(CNN) —  

On Monday, at least 21 people were shot in Chicago. Two of them – including a teenager – were killed.

The violent day comes just weeks after the nation’s third-largest city, long plagued by gun-related crime, marked its 15th consecutive month of declining killings and shootings.

Between January 1 and May 31, Chicago recorded 52 fewer murders and 229 fewer shootings – both a 21% drop – than during the same period in 2017, according to the Chicago Police Department.

City officials have attributed declines to hiring more officers, stronger community policing and investments in technology, such as ShotSpotter gunshot detection systems and predictive crime software that helps deploy officers.

While data suggest a downward trend in murders and shootings, crime tends to pick up with warmer weather, experts say.

“The warmth of summer encourages more time on the streets and sidewalks and more encounters among youths, which begin as verbal altercations, escalate to shootings and lead to homicides,” Arthur Lurigio, a professor of psychology and criminal justice at Loyola University Chicago, wrote Tuesday in an email to CNN.

“Rival gang members are more likely to interact in hot weather, roaming across arbitrary gang boundaries and into contested territories,” he wrote.

Police said the violence appeared to stem from an ongoing gang dispute.

Department records initially reported that 19 people were shot Monday but a department spokesman later updated the number to 21. Details of the two additional incidents were not available.

The latest incidents unfolded steadily Monday across a 25-mile swath of the city, from near the northern reaches of Lake Shore Drive to not far from the south suburbs, police records show. Map markers show the block on which police say the incident occurred, and times are approximate.

These were some of the shootings:

Incident 1: 12:20 a.m.

A 22-year-old man was transported to a hospital in critical condition after being shot in the chest and back while walking.

The victim couldn’t communicate with officers, and it’s not known where the shots were fired from.

No one is in custody.

Incident 2: 9:29 a.m.

A 45-year-old man was shot in the leg when someone approached his vehicle and fired multiple times.

The victim drove himself to a hospital and is in good condition.

No arrests have been made.

Incident 3: 2:16 p.m.

A man realized he’d been shot after he heard gunshots, then felt pain.

The 24-year-old was shot in the knee. He’s in stable condition at a hospital.

No one is in custody.

Incident 4: 3:02 p.m.

A 46-year-old man died after being shot in the chest five to seven times.

A woman who was riding in the car with him argued with “an unknown offender” just before shots rang out.

Incident 5: 4:12 p.m.

A 25-year-old man was shot in the chest. He drove himself to a hospital and is in serious condition.

Detectives are investigating.

Incident 6: 4:30 p.m.

A person was shot in the wrist as the occupants of two passing cars exchanged gunfire.

Incident 7: 5:45 p.m.

An 18-year-old man was shot in the leg and lower back while standing on the sidewalk.

An SUV pulled up next to the teen, and the occupants fired shots in his direction. The shooters fled.

No one is in custody.

Incident 8: 6:20 p.m.

An 11-year-old boy was shot in the lower leg while walking to the store. The shooter was also on foot, and the boy wasn’t the intended target.

He was taken to the hospital in stable condition.

No one is in custody.

Incident 9: 6:40 p.m.

A 21-year-old man was walking on the sidewalk when someone on a bike rode by and shot him in the upper back, then fled.

The man was taken to a hospital in critical condition.

No one is in custody.

Incident 10: 9:45 p.m.

A 46-year-old man was shot in the back while walking. His shooter also was on foot.

The victim was taken to a hospital in good condition.

No one is in custody.

Incident 11: 9:48 p.m.

A 30-year-old man was shot in the ankle while riding a bike. The gunfire may have come from inside a moving vehicle.

The victim made his way to a hospital and is in good condition.

Detectives are investigating.

Incident 12: 10:37 p.m.

A 22-year-old man was found in an alley with a bullet wound to his abdomen. He was taken to a hospital in stable condition, but police couldn’t provide details about the shooting.

No one is in custody.

Incident 13: 10:45 p.m.

At least three people were shot as they stood outside in a park.

A 27-year-old man shot in the leg is in stable condition. A 24-year-old man shot in the ankle is in good condition. A third victim was shot in the leg. A fourth person wouldn’t give any information.

No one is in custody.

Incident 14: 10:50 p.m.

Four teens were shot, one of them fatally, after a gunman fired from an alley.

A 19-year-old man died after he was shot in the chest.

A woman, 18, was shot in the thigh. A 16-year-old boy was shot in the foot and thigh. A 13-year-old boy was shot in the arm.

The three surviving victims are in stable condition.

No one is in custody.

CNN’s Ray Sanchez and Ryan Young contributed to this report.