Inside the big Diebold walk-in safe the air is dank with the memory of gangsters. Special Agent Oatess Archey rummages amid the M-16s and boxes of ammunition for a moment, then hauls out a black, oblong carrying case. It looks as if it might contain a trumpet, or a particularly complete set of socket wrenches. Mr. Archey opens it with care.
''I just had to show you this,'' he says.
Inside is a Thompson submachine gun, circa 1921. Archey picks up the snub-nosed gun and hands it to me. The weapon is surprisingly heavy, yet in this small room its menace seems diminished. It is as if we are visiting a prisoner in jail.
''This is a bit of history,'' Archey says. ''It was used in the St. Valentine's Day massacre. Eventually we're going to put it on display.''
At the FBI's massive Pennsylvania Avenue headquarters, relics of the past are everywhere. The public tour begins with John Dillinger's straw hat and La Corona cigar; it ends with a fiery demonstration of a Thompson, even though agents haven't used the ''Tommy gun'' for more than 10 years.
But beneath this veneer of G-man-era nostalgia lies a law enforcement agency whose personality has greatly changed since the days of J. Edgar Hoover. Once unwilling to chase more than bank robbers or kidnappers, the FBI is today moving after hard-to-catch Mafiosi and white-collar criminals.
''We're focusing on whole groups,'' said FBI Director William H. Webster in an interview. ''We're reaching up into the upper echelons of criminal enterprises. We're not stopping with street busts.''
Last year, for instance, 113 members or associates of La Cosa Nostra organized-crime families were convicted after FBI arrests. In Kansas City, ''Operation Strawman'' resulted in five mob leaders being found guilty of skimming profits from a Las Vegas casino. In New York, an assault on the five leading Mafia families has netted such figures as James Episcopia, a captain in the Bonanno mob.
Almost 100 members of organized crime's minor leagues - nontraditional mobs such as the Outlaws motorcycle gang - were also locked up last year as a result of FBI efforts.
''The bureau's gotten more arrests, more convictions, that go higher up than in the past,'' says a congressional crime expert. ''Before, you took off a few people at the bottom. Now you get a few in the middle, one or two at the top. Does it make a difference? I don't know. Is organized crime still operating? Yes.''
Director Webster admits that the FBI hasn't dismantled any mob family. But La Cosa Nostra, he says, is far less venturesome than it used to be.
''It's a mistake to say we can wipe out crime,'' he says. ''You've got to keep the pressure on all the time, make the response prompt, and assure as much as possible that crime will be punished.''
It is Webster himself, law enforcement experts say, who deserves much of the credit for the ''new'' FBI.
A former federal judge in St. Louis, Webster is approaching his sixth anniversary as the top cop in the United States. He prefers the honorific ''Judge'' to that of ''Director,'' and indeed there is a judicial air about him. He weighs words carefully, appears somewhat grave, and altogether seems ready to don a black robe at moment's notice.
''Webster is really a classy guy,'' says James Fyfe, a criminal-justice professor at American University.
If there is a catch phrase that characterizes his law enforcement priorities, it is ''criminal enterprise.'' In the past, says Webster, federal crimes were usually committed by free-lancers, such as John Dillinger, who acted independently. Today, the FBI director says, major criminals are more likely to be members of illegal, businesslike enterprises. Call it the dark side of entrepreneurism.
''It takes a major operation to be a major drug cartel,'' Webster points out. ''It involves arrangement for acquisition in foreign countries, transshipment to the US, lines of distribution around the country, and vehicles for laundering funds.''
Besides the 24 Mafia ''families'' scattered around the country, major criminal enterprises now in the US include Colombian cocaine rings, the Japanese Yakuza mob, prison-based groups such as the Aryan Brotherhood, the United Freedom Front and other terrorist organizations, and motorcycle gangs.
In particular, the big motorcycle gangs - Hell's Angels, Pagans, Bandidos, and Outlaws - are rapidly increasing the scope and sophistication of their crimes, Webster says. They now control more than half the illegal amphetamine traffic in the US, according to one Justice Department estimate.
''In many ways the motorcycle gangs are like a new generation of criminal groups picking up the crumbs (left by the Mafia) and starting to become important themselves,'' he says. ''They're already operating on the fringes of traditional organized-crime sources of funds - massage parlors, prostitution, that sort of thing.''
But the US public is probably most concerned right now about another of the criminal enterprise types - terrorists. Television footage of Middle East rubble has made many Americans aware of the damage that can be caused by a determined bomber with a truck.
''We should not allow ourselves to be stampeded by events which have not occurred,'' says Webster when asked about the threat of Mideast terrorism in this country. The overall number of terrorist incidents here, he notes, has dropped in recent years. As yet no Arab guerrillas have surfaced in the US - although Webster says the FBI last month headed off what could have been a major Mideast-related incident. He declines to give more detail.
''We've been getting, as we have for several years, intelligence information suggesting that groups are already here, will be coming, are ready for the word, and so on,'' says the FBI director. ''We can't discount any of that.''
Aided by some changes in law that ease investigation of organizations, Webster has set many of his agents on the trail of criminal enterprises. This year, almost half of FBI agent man-hours will be spent fighting organized crime, white-collar criminals, and domestic terrorism. Only 10 percent of agents' time will be devoted to personal crimes, such as bank robberies and kidnapping.
But fighting muscular criminal groups can be a dirty business. Some are wealthy enough to dangle tempting bribes before law officers - reportedly one reason J. Edgar Hoover kept his agents out of drug investigations. This risk of corruption, says Webster, is one today's FBI must take.
And investigating enterprises ''also involves, perhaps more extensively than in the past, the use of sensitive techniques,'' he says, ''the informant, the undercover agent, and electronic surveillance.'' Indeed, authorized FBI wiretaps are up sharply over the last two years, though they haven't yet reached the heights of the Nixon era.
The use of these sensitive techniques worries a number of congressmen and civil liberties lawyers. The memory of FBI abuses - such as the compilation in the 1960s of a dossier on Martin Luther King Jr.'s extracurricular activities - dies hard.
Herman Schwartz, a law professor at American University, says some recent FBI forays against criminal enterprises are close to ''fishing expeditions'' - general snooping proscribed by the Constitution.
Other civil liberties advocates, however, speak about Webster himself in guarded terms of praise. ''He has taken the bureau in the right direction, gotten them out of domestic political surveillance, focused on organized crime, been beneficial from a civil liberties standpoint,'' says Jerry Berman, a lawyer for the American Civil Liberties Union. ''That's not to say the bureau doesn't make mistakes.''
A civil liberties source who asked not to be named puts it more bluntly: ''Our only concern is how long Webster's going to stay. Right now, (he) is the cork in the bottle.''
In recent months, Judge Webster has sounded to some observers as if he might be planning a return to his St. Louis-area farm.
He insists, however, that he now has no plans to leave his FBI post. ''Don't write me off too soon,'' he says.