MANCHESTER —A year after a rash of suspicious fires prompted a multi-departmental attack on arson in the city, the rate of fires being referred to the city’s fire investigation unit has dropped by more than a third.

“Last year, the total number of fires investigated by this office, and we are not called to every fire, was 139 fires,” Deputy Fire Marshal Paul Lennon said. “This year, right now we are at about 57.”

An arson task force was formed by the city fire and police departments, with assistance from state police and the state fire marshal. Officially, cases investigated as potential arsons are listed as being of incendiary origin. It can take weeks, or even months, to determine that the fire was set. Investigators use physical evidence, a “paper trail” of insurance and financial issues and detective legwork to make a determination that a fire is a case of arson.

“Half of all fires that are set are set by juveniles,” Lennon said.

The fire department’s Fire Prevention Unit recommends that property owners leave outside lights on at night and cut back vegetation growing near buildings to reduce the opportunity for mischief-inspired fires.

Foreclosed properties are sometimes inviting for thrill-seeking firebugs and present their own problems, since sometimes it is difficult to find the party responsible for securing the property.

“It’s really complex,” Lennon said of the vast web of lenders, the repeated sale of mortgages to new investors and the emergence of companies that exist to hold title to millions of mortgages.

The complexity means a lot of time has to be spent tracking down the entity responsible for securing empty buildings and making sure overgrown shrubs and weeds don’t create an attractive venue for arson.

“It takes a lot of legwork, and you’re dealing with it throughout the entire country,” Lennon said. “You talk to a manager and, with their case load, it’s very challenging to find a person to talk to.”

Last year’s spate of arson fires included a staggering 18 fires set during March and April, most of which, the New Hamsphire Union Leader reported, occurred in a small section of the West Side.

Lennon said the public plays an important part of getting arsonists off the street.

“It is very important, it aids us tremendously,” Lennon said. “Anything is going to help, no matter how small, every bit of information is important.”

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Bill Smith may be reached at wsmith@unionleader.com.