In the United States, only every second murder case is solved.
According to research by the organization Murder Accountability Project and the broadcaster CBS, the clear-up rate in 2021 was lower than in any other year since the 1950s.
At that time, investigators between Los Angeles and New York were able to solve about nine out of ten homicides.
In addition to fewer police jobs and a wave of violent crime that has been spreading in the United States in recent years, observers also blame the unsolved cases on the disturbed relationship between the police and residents of poorer neighborhoods.
In an atmosphere of mutual distrust, fewer and fewer people are willing to provide information about possible perpetrators.
"The relationship between police and citizens only works as a two-way street, but we as officers keep blocking it," Philadelphia Police Commissioner Danielle Outlaw recalled of the police brutality debate.
In the Pennsylvania city of more than 1.6 million people, about 550 homicides were counted in 2021, more than in any previous year.
Less than every second murder has been solved so far.
The study by the FBI Homicide Accountability Project, launched seven years ago in Virginia, shows that homicides involving white victims are solved far more frequently than cases involving black victims.
For 2020, the organization registered an arrest rate of almost 90 percent for white victims.
For African-American victims, the rate was just under 60 percent.
Outlaw, Philadelphia's first black female police chief, is now calling for a better relationship between investigators and residents of poor neighborhoods so that more murders can be solved.
"Just police work is not enough," she told CBS.