A woman walks past an artist’s rendition of a gravestone in Woodbridge, Va. Each gravestone had no name or birthdate and was placed near the scene of an actual homicide. (Victoria St. Martin/The Washington Post)

There was the 14-year-old boy who was bludgeoned to death with a hammer. There was the youth counselor who police say rushed to the apartment of a 19-year-old client hoping to help — only to be sexually assaulted and strangled. And there was the rookie police officer who was gunned down while responding to a ­domestic-violence call just a few hours into her first day on patrol.

As law enforcement agencies across the region compile their end-of-the-year crime statistics, the tally is particularly grim in Prince William County, where the number of homicides so far has more than doubled the 2015 toll.

There have been 22 homicides in Prince William in 2016, compared with 10 last year.

In many communities, the prospect of 22 killings in a year might not seem like a cause for concern. This year, the District has tallied 134 slayings, and neighboring Fairfax County has had 18. But the numbers in Prince William are the highest in the county since authorities began tracking them in 1975. The next-most-deadly year was 2006, when 16 people were slain.

The spike has frustrated law enforcement officials, caught the attention of criminologists and sent a tremor of uncertainty through the Northern Virginia county of 450,000 people about 30 miles southwest of the District.

Jose Quinteros, the father of a 17-year-old who died in a July fire, sits across the street from his Manassas home as investigators worked on it as a crime scene. (Victoria St. Martin/The Washington Post)

“I don’t know what it is, but something’s going on,” said Krystal Pham, whose 26-year-old sister, Linh Pham, was fatally shot Nov. 6 in a Prince William shopping center. “I think 2016 has been a very bad year for a lot of people.” The man charged in the killing is the father of Linh Pham’s child, her sister said.

Asked about the homicide rate, Police Chief Barry M. Barnard, a 40-year veteran of the county department who was appointed midway through the year, shook his head and said simply: “We’re always going to remember 2016.”

Experts attribute the increase to a range of factors, including the prevalence of handguns and the lethality of those weapons. They also cite gang activity and domestic violence.

Jay S. Albanese, a criminologist at Virginia Commonwealth University, said that what’s happening in the county may be part of a national trend: Homicides increased nationally by nearly 12 percent from 2014 to 2015.

Despite that increase, Albanese said, a longer trend line shows a drop in killings. “You are twice as likely to die in a car accident as you are to die from homicide,” he said.

Albanese said that other factors, such as drug markets or gang activity, might also be driving increases in some areas, and those forces have played a role in Prince William. Three people, including members of the lethal MS-13 street gang, were arrested in connection with the county’s first homicide of 2016, that of Oscar Rene Andrade, 29, who police say was also robbed. Nine people, including some with ties to MS-13, were also arrested in the Oct. 29 shooting death of Edwin Ivan Chicas, 22.

Domestic violence is believed to have played a role in at least three of the county’s homicides in 2016, including the shooting deaths of Officer Ashley Guindon, 28, and Crystal Hamilton, 29. Guindon was responding to a call involving Hamilton when both women were killed. Hamilton’s husband, Ronald Williams Hamilton, 33, was charged with murder in their deaths.

A sadness still hangs in the air inside county police headquarters in Woodbridge. Every day, the chief wears a black bracelet that has a thin blue line and Guindon’s name.

Barnard said his detectives have made arrests in 18 of the 22 slayings. One was an apparent murder-suicide and three others remain under investigation.

Two of those killings involve a double homicide authorities have linked to an early-morning fire in the Manassas home of Jose Javier Avalos, 17. Avalos died in the July blaze, as did a 36-year-old man who relatives said was a family friend.

“I saw the picture of my friend and my son. I started crying [and] saying, ‘My son! My son!’ He’s the only one,” said the teen’s father, Jose Quinteros.

The county fire marshal’s office arrested a man who authorities said was spotted near the house with burn injuries. That man was charged with arson, but police are continuing to investigate the deaths, a department spokesman said.

Each of the slayings has stayed with Alexis Gomez. So much so that the county resident and artist molded copies of gravestones out of cement and placed one near the sites of homicides this summer. For his street art project, he chose the hashtag #SAVEPWC.

“The number seemed pretty unfathomable before it happened — even when it was at 11,” said Gomez. “And to get to 22, I’m still trying to hone in on 11. It’s pretty shocking.”

Corey A. Stewart, chairman of the Prince William Board of County Supervisors, said the supervisors have asked the police chief to make a formal presentation in the new year about the increase.

“Is there something underlying this spike in murders, or is this a coincidence we had a number of them this year?” said Stewart, a Republican with an at-large seat who recently announced his candidacy for governor.

Katheryn Russell-Brown, a criminologist and law professor at the University of Florida, said it feels as if people “agree to disagree” less often in the world today.

“The climate is very black and white, or very cut and dry — we’re less tolerant,” Russell-Brown said. “We don’t want to talk it through, we don’t want to work it out.”

Barnard noted that most homicides are committed by people who know each other.

“That does not at all for one moment diminish the tragedy,” he added. “Victims are an unbelievable loss to the families and friends, but it’s information for the community to know so that they can evaluate that themselves and make some decisions about how they go about their lives here in Prince William County.”

Jennifer Jenkins contributed to this report.