Serious crimes in Suffolk increased slightly in 2015 over the prior year, due largely to crimes that are also an issue regionally, according to Suffolk Police Chief Thomas Bennett.

Part I crimes — a Federal Bureau of Investigations classification — increased 4.7 percent. The designation includes homicide, rape, robbery, aggravated assault, burglary, larceny, motor vehicle theft and arson.

The largest percentage rise was in arson, which went up by 10 offenses — more than double 2014’s total.

“This represents an increase in arsons associated with vehicle thefts in neighboring cities where the vehicle was driven to Suffolk and burned,” Bennett wrote in an emailed response to questions. “There has also been an increase in fires set in vacant structures.”

Another large increase was motor vehicle theft, which went up 26 percent.

“There are several theft rings operating in Hampton Roads that have contributed to this increase,” Bennett wrote, noting that he could not comment further because of ongoing investigations.

Robbery saw a 19-percent increase, or a growth of nine offenses, Bennett also noted.

“This is a result of increased business robberies,” he wrote. “This trend has also been documented in many other jurisdictions.”

By actual offense numbers, the largest increase was larceny, which rose 86 offenses or 4.7 percent, Bennett said.

“This represents an increase of thefts from commercial stories and thefts from unlocked vehicles throughout the city,” he wrote. “Our citizens need to secure their cars at all times to prevent these thefts.”

Burglary and aggravated assault rose slightly, by 1.8 percent and 1 percent, respectively.

There were some declines in the individual crime categories. Rape fell by nearly half. Bennett could not pinpoint a reason for the decline.

And homicide fell 33 percent, from three offenses in 2014 to two in 2015. One of the 2014 offenses, however, was a vehicle accident that was charged as negligent homicide.

Three arrests have been made in the murder of Donta Williams, a 20-year-old from Portsmouth whose family reported him missing in January. His body was found in a lake near Lake View Parkway on March 9.

However, police continue to investigate the case of William Howard Batchelor, a 68-year-old Suffolk man whose body was found in a room at the Econo Lodge on Holland Road on Oct. 30.