Arson is the fastest-growing and most costly of all major crimes, yet it gets little attention, inadequate investigation and few convictions, according to a study for the Law Enforcement Assistance Administration.

In 1975, arson losses totaled an estimated $1.4 billion, up 325 per cent in 10 years, the study said, describing both the cost and the rate of increase as larger for arson than for rape, robbery, murder, burglary or other FBI index crimes.

Arson in 1975 also accounted for the deaths of about 1,000 persons, including 45 firefighters, and 10,000 injuries, the study said.

The study, conducted by Aerospace Corp, under a $90,000 LEAA grant analyzed arson statistics from 108 cities over four years. The report's summary said:

"Although arson is a felony, it is not included in police crime statistics because, in most states, the fire service has the responsibility for arson detection and investigation.

"Since arson cases are characterized by both lack of witnesses and devastation of the crime scene, the arson investigator faces difficulties not posed by other types of crime."

In the study's sample, revenge by adults and vandalism by juveniles are cited as the most frequent causes of arson. Fraud was the motive of only five percent of the arsonists studied, although it was involved in 17 per cent of a sample of arson cases.

A group of reporters for The Washington Post recently discovered that in New York, Chicago, Detroit, Philadelphia and Trenton, residents feel that a large number of fires set by juveniles are at the behest of landlords seeking to collect insurance on dilapidated ghetto buildings.

The LEAA attempt to assess the characteristics and motives of arsonists was impeded by a scarcity of arrests and convictions.

Fires classified as incendiary or suspicious, the study said, yield only nine arrests, two convictions and 0.7 imprisonments per 100. For the FBI's index crimes, the comparable figures are 21 arrests, six convictions and three jailings.

The study attributes this to a shortage of trained arson investigators, frequent lack of witnesses, destruction of physical evidence in fires, and confusion about the responsibility of police and fire departments.

Determined law enforcement efforts can curb the incidence of arson, the study found. In cities ranking in the upper third for arson arrests, there were 22 percent fewer arsons than in cities ranking in the bottom third, it said.

But law enforcement is hampered by a unique characteristic of arson: large-scale investigations may be needed just to determine whether there has been any crime at all.

The study recommends increased training for investigators, a nation-wide data system for law enforcement officials, and development of a flammable vapor detector to help in determining whether suspicious fires were deliberately set.