Washington — The radical Weather Underground, blamed for the recent Brink's robbery in Nyack, N.Y., has emerged from its hiding place at the very time when signs of domestic US terrorism had almost vanished.
Terrorist bombings that once occurred at a rate of 100 a year dipped in 1980 to 20. Total terrorist-related incidents also have dropped. In the first seven months of fiscal year 1981, the Federal Bureau of Investigation (FBI) recorded only 16, and most of these involve extremists with ties to other countries, ranging from the Irish Republican Army to anti-Castro Cuban groups and the Croatian National Resistance.
Homegrown radical groups, such as the Weather Underground which violently protested the Vietnam war during the '60s and early '70s, have faded from the public eye during the last five years. The FBI even had dropped its fugitive warrants for most of the members who were charged during violent antiwar protests. It was no longer even looking for Katherine Boudin when she was arrested last week in connection with the $1.6 million Brink's heist.
During recent years the FBI has gathered no information on the Weather Underground, says Joseph A. Valiquette, spokesman for the New York FBI office. Now that the group has resurfaced, a task force of FBI agents and New York police is combing through the evidence, seaching for signs of a terrorist network.
So far the team of investigators has turned up some links among nine persons arrested in the robbery and in the probe that followed it. Of the four caught immediately after the holdup, three were in the Weather Underground and a fourth was a member of the Black Liberation Army.
As evidence of a terrorism network, they found that Marilyn Buck, now a fugitive whose car was said to have been used in the robbery getaway, had rented several New York apartments as radical hideouts. Searching the homes, officials found weapons, disguises, and radio equipment. They also discovered literature of the May 19th Coalition, which the FBI says has supported the Weather Underground, the Black Liberation Army, and the Puerto Rican terrorist group FALN.
As the FBI spokesman concedes, such evidence does not draw a conclusive picture of a national conspiracy. However, it has set off speculation in Washington about a rebirth of the radical movement of a decade ago and adds new fuel to the debate over whether the federal government should step up its antiterrorist efforts.
''Before the Brinks robbery, it was considered paranoid to think that the Weather Underground was well connected'' or a serious threat, says Robert Kupperman, author of a recent book on terrorism. ''Today there's a kind of frenzy going on, led by the FBI.''
The truth lies in between, according to Mr. Kupperman, an executive director for the Georgetown University Center for Strategic and International Studies. He sees a loosely knit underground movement in the US, but he rejects the notion of the '60s groups making a comeback in the 1980s. The adage that '' 'You can't go home again' applies to terrorists,'' he says. ''The Weather Underground is not terribly viable.
''This incident will provide an excuse to claim that there are overwhelming ties'' among terrorists in the US, he says. ''That I do not believe.''
However, Kupperman warns that if a protest movement against Reagan economic policies grows into civil disobedience, that would be a ''mask'' for terrorists, including the FALN, Palestinian extremists, and Libyan nationals. He argues that US police are ill-equipped to handle massive disorders and should be trained in riot control.
Samuel Francis, a former researcher for the ultraconservative Heritage Foundation and a staff member of the Senate subcommittee on terrorism, sees the Brink's robbery as a clear sign of a ''resurgence of terrorism.'' He says that it shows the Weather Underground had graduated from bombing, which he maintains is relatively simple, to a complex robbery and that radical groups are joining together.
''It may be a last gasp now that they were caught. I don't think it was intended as a last gasp,'' he says, predicting that various terrorist groups will form tighter alliances. Mr. Francis, along with other conservatives, is pushing for lifting 1979 restrictions that forbid the FBI to investigate groups unless they are involved in crime.
Morton Halperin, director of the Center for National Security Studies, sponsored in part by the American Civil Liberties Union, said this week of the Brink's robbery, ''I don't think it justifies any change'' in FBI regulations. Declining to speculate on possible reaction in Washington, he added, ''I think everybody's just waiting.''
Rep. Don Edwards (D) of California is doing more than waiting. ''There's always a danger of an overreaction,'' he says. ''People are just waiting to jump in in a panic and chip away at our liberties.
''It's a run-of-the-mill Brink's robbery with rather dramatic undertones because of the history of these people. If there are any international implications, the FBI will handle them.''
''We've been monitoring terrorism in the subcommittee for a long time,'' says the California lawmaker. ''And we can certify that terrorism within the US is rather well under control by the FBI and local police. The incidents are decreasing, not increasing.''
''(FBI Director William H.) Webster and his people have respect for constitutional rights, and we can trust them to handle it,'' says Congressman Edwards.