Before last Monday’s mass killing at the Washington Navy Yard, the District’s homicide count was about on pace with 2012, a year that ended with 88 slayings, the fewest in a half-century.

But a lone gunman who opened fire in one building on the Southeast Washington waterfront has made it unlikely that the city will count record low homicides again this year.

The 12 killings — and three other slayings this week that have been all but lost amid the carnage at the Navy Yard — pushed this year’s homicide count to 82, compared with 64 at this time in 2012. D.C. Police Chief Cathy L. Lanier added the Navy Yard slayings to the District’s crime statistics but added an asterisk marking them as an exceptional tragedy. The city’s homicide count jumped 28 percent.

As more cities across the country join the grim list of those that have experienced mass-casualty attacks, authorities are struggling with how to record them for statistical purposes and present them to the public in proper context.

In big cities and small towns alike, leaders often cite murder rates as a measure of safety in communities. But mass attacks don’t comfortably fit into categories designed for killings during holdups, drug deals or domestic fights.

When numerous people are fatally shot at movie theaters, in schools and in offices, it skews annual statistics, instantly turning what could have been a low-crime neighborhood into the statistically most dangerous. Some officials argue that the spikes artificially inflate violence and don’t accurately reflect crime trends. Some places keep such tragedies off their lists, often because federal authorities took over the investigation, or because they consider them acts of terror instead of crime.

Authorities in Aurora, Colo., held meetings filled with soul searching to come up with a way to record last year’s killings of 12 patrons at a downtown movie theater. On the police crime chart, they listed the total number of homicides at 29 — including those who were killed in the mass shooting — but also put a number 12 in parentheses next to the total. In the town’s annual report, police explained that the victims claimed by one gunman turned what would have been a 3.5 percent drop in violent crime into a 1.9 percent increase.

“We didn’t want our tragedy to be a reflection of our crime rate,” said Sgt. Cassidee Carlson, a spokeswoman for the Aurora Police Department. “It’s very clear that what happened is not typical. Obviously, it needed to be reflected. It happened. But we wanted to be smart about how we reflect what happens in our city.”

Police in the District, unlike in other cities, handle the investigations of all homicides, whether they happen on a city street or in a federal park. Authorities knew the Navy Yard shootings would alter the year’s statistical landscape on crime.

“We had the lowest number of homicides last year in 51 years,” said D.C. Mayor Vincent C. Gray (D). “Obviously now we’re not going to be able to make that this year unless some miracle happens.”

Chuck Wexler, director of the Washington-based Police Executive Research Forum, said mass killings need to be honestly reported. Of the Navy Yard shootings, he said: “It’s a horrible tragedy, but it is what it is. They’re homicides. You report them, but with further explanation. ”

He called such violence a “sad commentary on where we are as a society, but I don’t think we do ourselves any favor by not reporting them. You’re saying, ‘Yes they died, but they died under very unique circumstances.’ You want to make sure they don’t go unnoticed.”

Criminologist James Alan Fox, of Northeastern University in Boston, said crime statistics are used “as a barometer of safety and of police performance” and said mass shootings should not be left out.

Police in Blacksburg, Va., list zero homicides for 2007, the year a gunman fatally shot 32 students, faculty and staff members on the campus of Virginia Tech, located within the city limits. The shootings are noted in Virginia Tech’s campus crime report.

Killeen, Tex., does not list the 13 people killed at Fort Hood in 2009 because it occurred on a military base, but it did count the 23 people killed in Luby’s cafeteria in 1991. Oklahoma City, however, recorded the 168 people killed at the Alfred P. Murrah Federal Building in 1995, nearly doubling the state’s yearly homicide count from 226 to 400.

Boston is not including the three people killed in this year’s marathon bombing as city homicides, saying they were victims of terrorism, not typical crime, adding that the case was investigated by the FBI.

Cheryl Fiandaca, a Boston police spokeswoman, said the three marathon bombing victims were killed as a result of terrorism. “I believe that when they take jurisdiction, they take the statistics,” she said of the FBI. “It was a terrorist attack. I’m not sure that fits the criteria of a city or street crime.” Cambridge, Mass., is counting as a homicide the death of an MIT police officer shot by the bombers three days after the attack in Boston. It was the city’s second so far this year.

The deaths in New York, at the Pentagon and in Pennsylvania on Sept. 11, 2001, presented a unique challenge to officials. Despite more than 2,800 deaths at the World Trade Center, including scores of firefighters and police officers, the official homicide count for New York City in 2001 is 649. Arlington County, where more than 180 people died at the Pentagon, lists three killings for that year.

The FBI’s Uniform Crime Report — a yearly compilation of crime statistics voluntarily submitted by cities and towns across the country — created a new category for the Sept. 11 deaths, noting in a special section that gives a statistical analysis of the victims that the authors “struggled with how to report the data to the public.”

The authors said the 3,047 total deaths are considered murders and non-negligent manslaughter but “are different from the day-to-day crimes committed in our country.” They said “combining these statistics with our regular crime report would create many difficulties in defining and analyzing crime as we know it.”

Sometimes the statistics are difficult to find. Littleton, Colo., is the town associated with Columbine High School, where 13 people were gunned down in 1999. But the police department lists one homicide for that year. The school is actually in unincorporated Jefferson County, which lists the deaths on its crime sheet.

But Newtown, Conn., Police Chief Michael Kehoe said he felt no need to explain the December shootings of 20 children and six staffers at Sandy Hook Elementary School, along with the shooter’s mother, which were the town’s only homicides last year.

“I think our community knows this is a very safe place,” Kehoe said. “I haven’t thought of putting an asterisk on any number. I think it’s well understood what happened here.”

Aaron C. Davis contributed to this report.