German Ambassador Heinz Kuhna was seated in his second-floor study here on May 16 when a rocket-propelled grenade exploded in the next room and nearly knocked him from his chair.

The attack came a week after a similar attack on the Dutch ambassador and was at least the fifth carried out this year by a terrorist group known as November 17, which has operated in the Greek capital for more than two decades.

Since November 1975, when the group assassinated the top CIA official in Greece, Richard Welch, the United States has pressed Greek police to bring the members of the group to justice, but officials here say their first arrest is still a long way off.

The group's long record of violence helps explain why the State Department spends more on diplomatic security here than anywhere else in the world, according to U.S. Embassy officials. In 24 years, November 17 attacks have killed 22 people--four of them U.S. government officials--and wounded 70.

November 17's seeming immunity has provoked renewed concern among U.S. officials that some Greek political and security officials do not share Washington's goal of bringing the group to justice. Instead, they complain, some within the Greek government may have sought to preserve its anonymity, possibly to hide past links to the organization by top Greek officials.

"There is a high degree of unhappiness in the U.S. government and this embassy about the fact that no one has been arrested" in 24 years in connection with the deaths of the four U.S. officials and the wounding of 30 others in attacks by November 17, said U.S. Ambassador Nicholas Burns.

Burns said he is confident that Prime Minister Costas Simitis and Minister of Public Order Michalis Chrysohoides share the American desire to act against the group. But a U.S. official who asked not to be identified said Washington could not vouch for "people at other levels [of government] who have potential associations" with the group. "There is a clear record of failure" so far, the official said.

U.S. officials say they suspect that arrests of group members have been blocked by both a lack of official interest and active opposition within the Athens government. Senior Greek security officials have rejected the accusation.

Burns said the issue is certain to be discussed during President Clinton's visit here Nov. 13, when security precautions are likely to be among the tightest for any foreign visit Clinton has made. U.S. officials say they hope the two NATO allies can finish drafting a protocol authorizing cooperation between a special squad of Greek police and a trio of FBI counter-terrorist specialists. The United States also wants to help train and equip Greek police involved in the investigation.

Washington's interest stems not only from the group's killing of Welch and the three other officials in 1983, 1988 and 1991, but also from its 1987 bombing of two buses carrying U.S. Air Force personnel; its attempted assassination of two other U.S. officials; a failed 1996 missile attack on the U.S. Embassy; and its attacks on many prominent U.S. corporations here.

November 17 also has killed more than a dozen prominent Greeks, but the group's actions have provoked only limited outrage here, partly because its overtly leftist and nationalist leanings and its strident anti-Americanism resonate with many Greeks. Greece was the site of furious demonstrations against NATO's bombing of Yugoslavia earlier this year, and Washington's image here is still tainted by its support for a Greek military junta from 1967 to 1974.

The group's name is taken from the 1973 date on which Greek army tanks smashed a student protest at Athens Polytechnic University. Its initial aims were to promote a Marxist-Leninist system, challenge U.S. influence in Greece and overturn an amnesty given policemen involved in torture.

U.S. and Greek officials say the group's membership was likely drawn from the same anti-junta student organizations that also gave birth to the Panhellenic Socialist Movement (Pasok), which ruled the country from 1981 to 1989 under Prime Minister Andreas Papandreou, a harsh critic of U.S. policies. The party rules today under Simitis, a political moderate and pragmatist who had been Papandreou's top economic adviser.

"It is logical to assume that people didn't want to look under every rock because of what they might find," a U.S. official said. "If they arrest the leader, for example, and he turns out to be a former best friend of a Pasok leader, that would be embarrassing."

Some prominent Greeks share U.S. suspicions. "There were a lot of cases where police did not do what they had to do," said Dora Bakoyannis, whose husband, Pavlos, was assassinated by November 17 in 1989. "Everybody in Greece knows that. So the question is why? Did they have someone inside? . . . There are a lot of heavy accusations behind closed doors, but we don't have anything tangible," said Bakoyannis, whose father is a former prime minister.

Mary Bossis, a former Public Order Ministry official and the author of several books on terrorism, said: "We are very gossipy . . . and here we have a group where nobody seems to know anything." If Greek security agencies could keep 21 million files before 1981 and penetrate hundreds of communist cells in that era, why can it not find November 17, she was asked. "If they want to find them, they will," she said.