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Four decades.

That’s how long it took police in California to identify and, on Wednesday, April 25, arrest Joseph James DeAngelo, 72, below, the man they believe is the Golden State Killer, who was linked to at least 12 homicides and 50 rapes in the 1970s and ’80s. While law enforcement, and victims of the killer, welcome the apprehension, it raises a question: How many killings go unsolved?

Homicide clearance rates, 1980-2012

The map below offers a state-by-state breakdown of the percentage of slayings solved, according to the Murder Accountability Project.

California had the highest number of reported homicides in the period, 113,762. The state with the second-most was Texas, with 70,807. New York followed with 57,641.

The annual average of reported homicide cases for California in the period was 5,125.

We’re getting worse at solving homicides

The clearance rate has fallen from 91 percent in 1965 to about 59.4 percent in 2016.

In the same period, the U.S. population increased by 54 million and the homicide rate per 100,00 declined from 7.4 in 1996 to 5.3 in 2016.

Homicides by county, 1996-2016

The map below is from the Murder Accountability Project’s collection of data on a county-by-county basis. The circles indicate the number of killings and methods used to kill. The totals can be found online at murderdata.org.

Highway killings

Due to an abundance of unsolved killings along the nation’s highways, the FBI announced the Highway Serial Killings initiative in 2009 to raise awareness among law enforcement agencies and the general public.

(Courtesy of the FBI)

Analysts created a national matrix of more than 500 homicide victims from along or near highways, as well as a list of some 200 potential suspects.