Prince George’s County Police investigate the fatal shooting of a man and wounding of a 2-year-old on Dec. 19 in Seat Pleasant, Md. Homicides declined in 2012 in Prince George’s and the District. (Mark Gail/THE WASHINGTON POST)

The number of homicides dropped again last year in the Washington region, including in the District, which grew in population and yet recorded the fewest killings in a half-century.

As of Monday evening, the District had 88 killings in 2012, a milestone for D.C. Police Chief Cathy L. Lanier, whose long-standing goal has been fewer than 100 homicides. As recently as 2009, the District had 140 killings. In 2011, there were 108.

In Prince George’s County, where crime dropped in nearly every category, there were 63 homicides, down from 97 in 2011, a harkening back to the 1980s, when the county had more farmland than urban centers or upscale subdivisions.

Police point to several reasons for the decrease, which has been part of a years-long national trend in some major cities across the country.Authorities have broken up dozens of violent gangs, seized thousands of guns, used technology to monitor the streets and directed additional resources to high-crime areas.

And District officials have given residents a financial incentive to provide tips: nearly $600,000 in reward money was paid out last year.

Although homicides have fallen, robberies have been a stubborn problem in the District. Smartphones and designer apparel are popular targets. Police say some criminals are committing robberies for quick money as the area’s drug trade, once a chief driver of the killings, has declined.

“We used to have 11 shootings and seven, eight homicides in a weekend,” Lanier said in a recent interview. “Now we’ll have a whole weekend where we don’t respond to a shooting.”

Killings in some of the region’s suburbs, where violent crimes are far more infrequent, remained about the same as the previous year, with Montgomery County recording 15 and Alexandria none. A few counties posted increases: Fairfax had 17 and Arlington had four, including jewelry store owner Tommy Wong, who was slain inside his shop. In 2011, there were 11 homicides in Fairfax and none in Arlington.

In the District, which has added more than 30,000 residents in just over two years, gun assaults also decreased about 11 percent last year, continuing a recent trend. But overall, assaults with deadly weapons have increased about 7 percent, police data show.

One of the most widespread crimes in recent years has been street robbery. At the beginning of the year, holdups spiked so severely that the department turned its focus to curbing them, beefing up its robbery unit and deploying extra patrols and teams of undercover decoys.

The numbers stabilized by the end of the year, but there were still some high-profile and brutal robberies, including the case of Thomas Maslin, who was beaten with a bat and left near Eastern Market. The assailants were after Maslin’s phone, police said.

Robberies generally decreased last year in the suburbs, including in Arlington, Alexandria, Prince George’s and Fairfax, which had a 20 percent decline in holdups.

Criminal homicides in the Washington area in 2012.

New York City announced an all-time low in homicides this year, with 414 as of Friday, 100 fewer than in 2011. The city also posted a record low number of shootings, 1,353, down from 1,420 the previous year.

Chicago, however, is experiencing a wave of violence and recently recorded its 500th killing of 2012.

Homicides in Philadelphia and Baltimore remain down significantly from historic highs, but were up slightly compared with 2011.

In the Washington region, there were several particularly shocking killings in 2012. In August, an armed intruder killed 17-year-old Amber Stanley in her Upper Marlboro home. Not a month later, another high school student, Marckel Ross, 18, was gunned down as he walked to Central High in Capitol Heights.

In the District on Christmas Eve, Capitol Hill resident Jason Emma, 28, was fatally shot near his home in what police think was a robbery. Earlier in December, Selina Brown was gunned down by the father of her toddler daughter as she boarded a bus. The child, who was in Brown’s arms, was wounded in the shooting.

In the six years that Lanier has been police chief, homicides have declined by more than 50 percent. But during the interview, Lanier acknowledged that spasms of violence can still break out with alarming frequency and said that the homicide rate “has to go lower.”

Just this week, there were shootings within hours of each other in neighborhoods just south of East Capitol Street that left two men dead — Angelo Alphonso Payne, 23, and Darnell Rivers, 22.

In recent years, the drug trade has steadily declined as open-air markets have faded and criminals have turned to stealing and selling smartphones and other devices. Last year, there were fewer than 10 drug-related homicides, officials said — a far cry from the 1990s, when the trade in crack cocaine was much of the reason the District saw close to 500 homicides a year.

In 2007, Lanier said, there were 70 gangs involved in criminal activity. Now there are about 20. In 2008, there were 142 homicides involving guns. Last year, there were about 56 — a sign, police say, that there are fewer guns on the streets.

And several years ago the city paid out $200,000 for tips leading to arrests and convictions in homicide cases. Last year, it awarded $575,000.

“People walking around saying ‘we can’t prevent homicides’ is one of the most frustrating things,” Lanier said. “We can prevent homicides.”

Gang unit officers targeted certain areas and let gang members know that they were being watched. If a gang fight broke out in a school or on Metro, officers would be on members’ doorsteps hours later, telling them they would go to jail if they retaliated.

The department has emphasized technology. The District has the country’s biggest deployment of ShotSpotter gun sensor technology, which alerts police to the sound of gunfire. Lanier also oversees the department’s license plate readers, scores of cameras fanned across the city that spot wanted cars and monitor vehicles traveling in and out the District.

Police are testing a system that would beam real-time footage of shooting scenes directly to laptops inside patrol cars. The program connects ShotSpotters with the department’s 91 closed-circuit surveillance street cameras.

The hope, Lanier says, is that officers can use the technology to quickly track down shooters and witnesses. “It will be a tremendous asset,” she said.

The department also began working with businesses to educate them about operating their private surveillance cameras. The focus is on formatting, lighting and information storage to help the police pull footage quickly. Too often, police say, they go to a business after a crime and have trouble accessing or using camera images.

Lanier said the instructional help for businesses could turn thousands of cameras into useful police resources.

Homicides ticked up slightly in Fairfax, as the county saw one of the worst incidents of violence in recent years: In September, a Herndon father killed his wife and two sons before turning a gun on himself. Last week, Zavier O. Stringfellow, a 19-year-old college student and former high school football star, was found stabbed to death.

In Prince George’s, county officials attribute some of their success to a neighborhood initiative that turned government officials’ attention to six of the county’s most beleaguered areas. They demolished vacant apartment buildings, cleaned up run-down areas and offered social services to some residents.

“You can’t arrest your way out of crime problems,” said Prince George’s Police Chief Mark Magaw. “You’ve got to get at the core of what’s driving those issues.”

Peter Hermann, Matt Zapotosky, Clarence Williams, Justin Jouvenal, Michael Laris, Caitlin Gibson and Jeremy Borden contributed to this report.