Nassau and Suffolk counties each recorded two dozen homicides in 2015, with Nassau showing an uptick from 18 the previous year and Suffolk seeing a decrease from 29, police statistics show.

The homicide totals for both counties are down this year from their five-year averages. And across Long Island, overall violent crime — including homicides — fell last year compared with 2014 — about 5 percent in Suffolk and about 6 percent in Nassau.

The killings in both counties were fueled primarily by gangs, domestic violence and drugs, officials said, and they were not limited to historically crime-ravaged areas.

In Nassau, for example, many of the homicides were outside the gang-dominated area referred to as The Corridor — a zone that includes Uniondale, Roosevelt and Hempstead, which together were hit with 10 of the county’s 24 homicides.

Homicides also were reported in Mineola, Jericho and New Hyde Park.

A variety of weapons

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The accused killers used a variety of weapons, including guns, knives and — in one instance — a dumbbell. In one case of domestic violence, an 18-month-old boy was beaten to death. In general, motives ranged from street beefs to personal grievances, police said.

Illegal guns remained a strong force behind many of the homicides, records show.

Police officials in both counties pointed to robust anti-illegal-gun initiatives, but noted an almost constant flow of arms from Southern states, which can be difficult to contain.

Officials said about 45 percent of Nassau’s killings resulted from gunfire; 35 percent of Suffolk’s were from shootings.

Domestic violence accounted for a large portion of the homicides in both counties. In the most recent killing just four days after Christmas, Jordan Johnson, 24, fatally stabbed his father, Russell Johnson, 55, in their Freeport home during an argument, police said.

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Authorities said two other killings were committed by children against their parents: In June, Vaughn H. Bloom, 44, fatally stabbed his mother, Sandra Bloom, 67, in the neck inside their New Hyde Park home, police said.

In August, police said Suzan Grossman-Kerner, an attorney from Oceanside, fatally stabbed her mother, Irma Grossman, 79.

Police and lawyers have pointed to mental health issues as possible motives in all three cases.

Toll of domestic violence

In Wading River, police said, a murder-suicide left three people dead in February: Tanya Lawrence, 43; her daughter, Danielle Lawrence, 17; and suspected shooter and Tanya’s boyfriend, Thomas Calhoun, 44.

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In another domestic violence killing, Tricia Odierna, 42, of Lake Grove, was found dead in her home in October. Odierna’s husband, Paul Leitgeb, 49, a former NYPD transit officer, told police he had hit Odierna in the head with a baseball bat and then strangled her, officials said.

Still, Nassau’s increase in homicides could not be attributed to any single issue, said Det. Sgt. Patrick Ryder, commanding officer of the department’s Intelligence Unit.

“We are slightly up over the norm,” he said. “There’s nothing that’s contributing to it directly. When you look at the overall picture, it’s so spread out [geographically], there’s not one conclusion you can draw.”

Despite Suffolk’s decline in homicides, Deputy Police Commissioner Timothy Sini said the department is ramping up its efforts to focus on drugs, gangs and illegal guns, which he said were at the heart of much of the county’s crime, including homicides.

Atop Sini’s agenda, he said, is strengthening the department’s partnerships with federal authorities following the arrest of former Chief of Department James Burke, who is being held in federal custody without bail on charges that he orchestrated a wide-ranging cover-up after beating a drug-addicted man who took a duffel bag from his department vehicle.

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Sini, a former federal prosecutor whose permanent position as Suffolk’s police commissioner is pending approval by the county legislature, said in the coming weeks he plans to install a Suffolk police officer in the U.S. attorney’s office for New York’s Eastern District to work as a liaison to help identify cases in which federal charges — often with lengthier prison sentences — would be appropriate.

“It’s a greater deterrent,” he said. “It’s a good way to get guys to flip, so that they can help us build these long-term investigations in Suffolk County to help us get at the root of the gang problems.”

Although Suffolk’s final homicide tally for 2015 is lower than the previous year, the county went through a spate of homicides in 2015 in a short period of time.

In the first week of November, for example, there were three homicides in Suffolk. Since October, there have been 10 in the county.

One of deadliest in Wyandanch

One of Suffolk’s deadliest cases of 2015 involved three people who were fatally shot in Wyandanch in June. The killings, which police suspect may have been gang-related, have not been solved. Sini declined to comment on the status of the investigation.

In Nassau, perhaps the most notorious killing of 2015 was that of Dejah Joyner, 12, who was struck and killed by a bullet that blasted through her family’s living room window in Hempstead while she ate dinner. The case, which investigators think may have been the result of gang fighting, also remains unsolved. Ryder, commander of the Intelligence Unit, said he could not comment on specifics of the investigation, but said solving the case remains the department’s top priority.

In a crime spree that lasted months, the killing of gas station attendant Hany Awad, 56, of Levittown, who was found shot to death behind the counter of his brother’s BP station in Jericho in late January, was among at least 10 gas station and convenience stores held up at gunpoint.

Police have charged Joshua N. Golson-Orelus, 23, of New Cassel, in Awad’s killing.

Nassau and Suffolk to focus on robberies

Both departments pointed to reducing robberies as a major goal in the new year.

Sini said robberies, which were up 15 percent this year in Suffolk, are committed principally by drug addicts and gang members. He said the department plans to use a similar playbook deployed previously that helped cut larcenies and burglaries: a concentration on patterns, outreach to pawn shops and prisoner debriefings, which often can cull valuable intelligence.

Ryder said Nassau plans to focus on the only two major categories in which crime was up slightly — robberies (up about 1 percent) and assaults (up about 4 percent), which sometimes go hand in hand. The uptick was fueled, in part, by the increasing number of young people carrying expensive electronics, such as iPhones and designer headphones, he said.

“That’s a problem,” he said. “It’s something we need to go after and address.”

Suffolk solved 11 of its 24 homicides last year, while Nassau closed 14, officials said. The clearance rates of 45 percent in Suffolk and 58 percent in Nassau are below the national average of 64 percent.

Officials in both counties decried the ongoing difficulties in getting witnesses in many communities to cooperate with police for fear of retribution.

Sini said he expected Suffolk’s Homicide Squad to close more killings in the coming weeks, as almost half of them occurred in the final three months of the year. Of those 10 since October, three have been solved, Sini said.

Ryder said the department’s Homicide Squad is working diligently to close its open cases and has the reach of the entire department to get the job done.

“They get any resources they want,” Ryder said of the squad. “They get top pickings. They need extra help, they get it.”