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In his reelection campaign, Chicago Mayor Rahm Emanuel is taking credit for a slight decline in the city’s homicide rate. But a WBEZ investigation raises a question about the murders that are still happening: Is the city doing enough to put the killers behind bars?

Emanuel has allowed detective ranks to decline during his term even as internal police records show some of the lowest murder clearance rates in decades. Our story (listen above) explores those rates through the eyes of city detectives and a mother who lost her 18-year-old daughter in an unsolved case last October.

A few notes about the data (charted below): Regarding the detectives, the number on the payroll is down by about 19 percent since Emanuel took office, according to records obtained by WBEZ under the state Freedom of Information Act. The ranks of evidence technicians and forensic investigators have thinned by even larger proportions.

Detectives say the drops owe to regular attrition such as retirements and promotions. A police spokesman says the city is planning to add 150 new detectives this year. But they won’t make up for the attrition during the mayor’s term.

About the murder clearances, the department calculates the rate two ways. The simple way accounts only for cases closed in the same calendar year in which the murder took place. By that gauge, the police cleared 28.7 percent of last year’s murders. The other calculation — the one preferred by the city — includes clearances of murders committed in previous years, leading to a 2014 rate of 51.8 percent. By either measure, the city’s clearance rate is near its lowest level in decades. Chicago’s also doing poorly compared to other big cities, according to FBI clearance figures for 2013, the most recent year available.

Zooming in further, the term “cleared” means closed but not necessarily solved. In some cleared cases, the killer was not charged or even arrested. During Emanuel’s term, roughly a quarter of the murder cases the police have closed were “exceptional clearances” because, for example, the suspect had died or fled the country or because prosecutors had declined to bring charges for various reasons, including a refusal by witnesses to testify. Last year, 42 of 213 clearances were “exceptional.”