For Montgomery County fire fighters and insurance company officials, the trend is alarming: 650 deliberately set or suspicious fires last year caused $6,553,286 in losses; 755 arson fires set this year have already piled up $5,643,198 in losses.

County Executive Charles W. Gilchrist, citing those statistics, yesterday announced the formation of a 12-member arson task force, which will study the arson problem and make recommendations on how to combat it.

Arson has been called the nation's fastest-growing crime, and in Montgomery fire losses from that cause have more than tripled since 1977.

The perpetrators are mostly teen-agers, but also landlords and home owners, businessmen and pyromaniacs. The targets are schools, homes, apartments, cars and businesses. Motivation ranges from profit to revenge to mental illness. And investigators left to pick through the debris rarely can count on finding the telltale fingerprint or the eyewitness, or to hear the confession of a remorseful arsonist. Of the 139 known arsons in the county this year, only 29 of the cases have been closed, fire officials reported.

"It's a tough, tough crime to prosecute," said James F. Dalton, the county's chief fire marshal.

The Montgomery task force will be chaired by Donn Knight, a fire insurance executive with the Government Employees Insurance Co., and will include representatives from the fire and police departments, the public schools, the state's attorney's office, the business community and the insurance industry. The goal, Gilchrist said, is to coordinate the antiarson activities of those government and commmunity sectors most touched by arson, and to provide "a management approach to the problem."

"Like drunk driving, arson is a community problem," Gilchrist said. "You've got to bring the resources of the entire community together to solve the problem."

The task force idea has been tried in other cities with much success, according to John Lynch, the national program manager for the Federal Emergency Management Agency's antiarson program. Salt Lake City reported a 183 percent increase in arson prosecutions in the first year since the formation of its task force, Lynch said. Phoenix, with a similar task force, reported a 50 percent increase in arson convictions last year, and a 25 percent drop in arson. Some of the steps taken to combat arson are installation of telephone hotlines, juvenile education programs and provision for counseling.

"If I were an arsonist in Montgomery County, I would start looking to relocate," Lynch said. "Montgomery County is going to become a very hot place for arsonists."