The Chicago Police Department has issued a warning to its officers after three fatal street killings using high-powered military-style weapons.
Officers are finding themselves outgunned by gang members equipped with powerful assault rifles such as AR-15s and AK-47s.
The force has warned officers that such weapons are powerful enough to pierce bullet-proof vests.
There were 762 murders in Chicago last year – the highest amount in almost 20 years – with gang-related violence blamed for the sharp rise in homicides in a few of the city’s poorest neighbourhoods.
Police increased patrols after gang member Daniel Cordova was shot dead on Saturday morning.
The 26-year-old’s body was found between parked cars in the Brighton Park neighbourhood in the South Side area.
The following day, 10 people were shot during a makeshift memorial for Mr Cordova about a block away from where his body was discovered.
More than two dozen shots were fired in the mass shooting. Two people were killed and eight others injured.
The man and woman killed, who were brother and sister, were believed to be members of the same gang as Mr Cordova. They were shot with the same type of high-powered military-style weapon, police said.
A rival gang is suspected to be responsible for all three deaths.
In a separate shooting on Monday, two plain clothes police officers were injured after being shot at with a high-powered rifle while sitting in an unmarked vehicle.
One officer was shot in the arm and hip and the other in the back after seemingly being mistaken for rival gang members.
An 18-year old has been charged with attempted murder and aggravated battery with a firearm.
No arrests have been made in the suspected gang shootings at the weekend.