Homicides are down in Washington, D.C., but some of the area's larger counties saw an uptick in murders in 2016.

There were 135 homicides in the District last year, a 17 percent decrease, according to the Metropolitan Police Department's website. Of those murders, 70 percent have been closed.

But some of the District's most notable cases remain unsolved, including the murder of DNC staffer Seth Rich.

While the District has seen a slight decrease in homicides, Prince William County in Virginia and Prince George's County in Maryland are dealing with a notable increase.

Last year, 22 people were killed in Prince William County, which has 450,000 residents. There were 10 homicides there in 2015. Last year's homicides are the most the county has seen since it started keeping records in 1975, according to The Washington Post.

Prince William County Police Officer Ashley Guindon was shot and killed in February by a man who apparently shot and killed his wife before officers arrived. Guindon, who was 28, was only hours into her very first shift.

Prince William County Police Chief Barry Barnard said arrests have been made in 18 of the 22 slayings. He said most cases involved people who knew each other, but that doesn't "diminish the tragedy," according to the Washington Post.

A police officer was also among Prince George's County's 98 homicides in 2016.

Officer Jacai Colson, 28, was fatally shot by a fellow officer as a gunman attacked a police station in the county. Colson, who was dressed in plainclothes, tried to neutralize the shooter. He was shot when another officer did not recognize him.

Domestic violence was also a root issue in many cases in Prince George's County.

The county's domestic violence issue was highlighted in several high-profile cases last year, including a shooting spree that crossed into Montgomery County.

Gladys Tordil was fatally shot by her estranged husband, 62-year-old Eulalio Tordil, as she sat in the parking lot of a Prince George's County high school, police said.

Investigators say Eulalio Tordil then drove to Montgomery County, where he fatally shot two people during botched carjacking attempts.

Santa Lizama, Tarekka Jones and NeShante Davis were also killed as a result of domestic violence, Prince George's County police say.

Fairfax County in Virginia also saw a slight increase in homicides in 2016. Last year, there were 18 murders; 5 more than in 2015.

The most recent homicides in the county were reported just days before Christmas. Henok Yohannes and Kedest Simeneh, both 22, were found shot to death two miles apart.

Yohannes was found in his home while Simeneh's body was found in the backyard of a home in Springfield, Virginia. Police say their deaths are connected, but no arrests have been made.

But not every jurisdiction has seen an increase in homicides.

Homicides dropped 50 percent in Montgomery County, where 15 murders were reported in 2016.

See the homicide rates for the D.C. metro area in 2016 and 2015 below:

Washington, DC

2016 = 135

2015 = 162

Prince George's County

2016 = 98

2015 = 77

City of Alexandria

2016 = 7

2015 = 4

Arlington County

2016 = 1

2015 = 2 (Note: 1 homicide was an officer-involved homicide that was later deemed justified)

Prince William County

2016 = 22

2015 = 10

Fairfax County

2016 = 18

2015 = 13

Montgomery County

2016 = 15

2015 = 30

Loudoun County