Behind the quiet facade of a tranquil brick town house in Alexandria, terrorism runs rampant.

Or at least the thought of it.

"A well-planned assassination is almost impossible to stop," said Roy C. Tucker, 57-year-old president of Risks International Inc. The company, located smack in the heart of Old Town, publishes a monthly newsletter devoted to assessing the state of worldwide terrorism. among the 100 or so subscribers, who plunk down $960 a year for the latest trends on terrorism, are multinational banks, corporations and the U.S. government itself.

Tucker, a tall, bespectacled lawyer who retired in 1977 as commander of the Air Force's Office of Special Investigation, yesterday was asessing the assassination of former Iranian diplomat Ali Akbar Tabatabai.

"It was obviously a very well planned, well thought out operation," Tucker said, lighting a cigarette. "That wasn't an amteur job."

Some aspects of the crime would not suprise his subscribers. they know, for example, that morning attacks are favored over midday or evening: the weapon, a 9 mm handgun, is commonly preferred by terrorist teams involved in assassinations; that attack is most likely to occur during the normal day-to-day activities of the victim.

The killing was a typical, Tucker said, in the fact that Tabatabai was slain at his home.

"Usually, that's the least likely place to get it. the most likely is going to and from work. But since this man did most of his work at home, the killers knew his routine," Tucker theorized. "He was in the habit of staying home. They figured that was the best place to get him."

A man like Tabatabai, who had received numerous death threats, "you would have thought would have taken more precautions," said Tucker. "Maybe he was just careless."

Tucker puffed on his cigarette. "But hell, he's no bigger target than a lot of other people walking around."

Which is why terrorism has become big business. Risks International Inc. is just one of a growing number of American firms -- including armored cars, bodyguards and electronic surveillance -- cashing in on what has become a billion dollar growth industry.

"Are we in if for the money? Well, year, we got into it for the business. Terrorism has become a popular word," he said.

Tucker and Charles A. Russell, a former colleague in the Air Force's Office of Special Investigation, started the business two years ago. Tucker claims it has the country's first privately owned computerized data base on terrorist activities.

Armed with their experience, the two men bought a radio shack computer for $6,000 and started compiling the data.

Most of the information tucker said, is gleaned from newspaper reports, private sources and Reuters news ticker he set up in the back room.

"We're not spooks," Tucker said yesterday. "We're strictly librarians."

Each month, Tucker breaks down terrorist activity by country, city, nationality of target and type. In March, for example, 241 significant terrorist actions worldwide were recorded, an increase of 66 incidents over the 175 actions reported in February.

Of those 241 attacks, 44 percent were bombings, 27.8 percent were attacks on facilities, 26.1 percent were assassinations, 13 percent were kiddnapings, and hijackings and maiming were both .4 percent.

Countries also are broken down in assessing risks: "civil war areas" (Syria), "high intensity areas" (El Salvadro, Iran, Italy and Turkey), "middle intensity areas" (Colombia, France, Guatemala, Philippines and Spain), and "low risk areas" (Bolivia, Brazil and Greece)

A typical issue might feature such topics as, "Female Terrorists A Growing Phenomen," "Assassinations: The Deadly Tactic" and "Reagional Risk Assessment: Latin America."

"It's fun," said Tucker, assessing his own occupation. As for the newsletter, "compared to the National Enquirer, it makes pretty dull reading," he said.