The Gambetta apartment complex on Roustaing street in this working-class suburb of Bordeaux is a rather bland-looking beige rectangular block, with a parking lot out back and supermarket across the street. Many of the tenants are students living month to month -- which is why a landlord became suspicious last summer when one student gave him three months' rent in advance, in cash.

The student exactly fit the profile -- a normally hard-up person paying large sums in cash -- that France's regional anti-terrorism police had told apartment owners to watch out for. Such a person, they said, might well be an aboveground supporter of the clandestine Basque separatist group ETA, whose campaign to create a separate homeland for the Basque people has killed more than 800 people since 1968.

French anti-terrorist police placed the apartment under surveillance, and on Sept. 16 their stakeout netted a husband and wife who Spanish politicians say commanded ETA's military wing. Juan Antonio Olarra Guridi, known as "Otsagi," was arrested along with his wife, Ainhoa Mugica Goni, called "Olga."

They have been together for 15 years, living in the shadows of a world of shifting identities, fake documents, multiple aliases and constantly changing apartments and cars. But investigators have slowly gained insights into how such people operate, how they finance themselves and use France as a base for attacks in Spain. A handful of hardened "activists" -- assassins and their handlers -- can survive for years using a much broader network of aboveground sympathizers, like the cash-rich student, police have found.

Guridi, who was born in 1967, has been linked to several assassinations in Spain, according to French investigators and media sources, committing his first killing in 1991 or 1992, before rising through the ranks to become overall military commander. Investigators believe he was involved in the attempted assassination in 1995 of Jose Maria Aznar, then the opposition leader and now prime minister of Spain.

Goni, investigators said, was born in San Sebastian, Spain, and became a commando a few years after Guridi, in 1994. Goni is considered Guridi's equal in the ETA structure, investigators said.

At a two-day trial in Paris that ended Jan. 10, prosecutors demanded an eight-year prison term. Guridi offered little in his defense, saying only: "We tried several times to make peace proposals, and the solution will not come from a process like this one."

He is not charged with committing assassinations in France, but with using his base here to organize killings across the border in Spain. Sentencing is set for early February. Typically, Basque militants captured in France are sentenced to between eight and 10 years, and then transferred to Spain, where they can receive sentences of 20 years or more.

Guridi moved up to take control of ETA's military wing after the arrests of several predecessors. Investigators said that showed how quickly the organization can replace captured leaders and stay one step ahead of the police.

"You cannot break the network," said one investigator with many years' experience tracking ETA suspects. Guridi's arrest "will not cripple the organization."

Although ETA's attacks are carried out in Spain -- targeting politicians, journalists, judges, members of the security forces, and lately tourist spots like resorts on the southern coast -- the command-and-control centers are here in the Basque region of southwestern France, investigators say. Many Spanish Basques crossed over into France during the 1936-75 rule of Generalissimo Francisco Franco in Spain; to them, there is no border between the Spanish and French Basque areas.

"The base is here in France," said an investigator familiar with ETA's structure. "The logistical base is here. They form their military operations here in France. The large reserve of their recruits is here in France."

Officials said ETA often conditions new recruits psychologically for assignments in Spain by putting them in a darkened apartment for several days or weeks with no access to the outside world, no newspapers and no television.

The recruits usually come from Basque youth organizations and get their violent start by staging small-scale attacks or petty crimes. Investigators said that was how Guridi, a native of Donostia, Spain, began his career in ETA.

Investigators said the ETA structure exists on three levels. At the top, the smallest level, are the "activists" such as Guridi and Goni. This group is believed to number at least 100 people, and at times as many as 150. Many are related to each other, making infiltration difficult. And women play a key role, including committing assassinations, officials said.

The investigators said the activists often go abroad for training -- to Mexico, for instance -- where they meet up with rebels in the provinces. Other destinations are Central America and Algeria.

The second, larger level includes supporters and sympathizers who provide the armed operatives with logistical help. These people, such as Saroia Galarraga, the student who rented the Gambetta apartment for Guridi and Goni, usually keep clean police records, so they can move in the open to carry out such tasks as renting property, buying cars and opening bank accounts, all legally in their own names.

Galarraga used to lend them her car as well. She was arrested along with the couple when they came back from shopping at the supermarket across the street from the apartment.

Investigators said a third and important level of ETA sympathizers are the ones who exist in "the cultural milieu," perhaps hosting concerts or other events in Spain and France to raise money that can be funneled to the activists.

Residents of the Gambetta apartments said they saw little out of the ordinary about the unit rented for the couple, except that it appeared to be used only rarely. Inside, according to neighbors, police and local media reports, it appeared to be a typical ETA safe house for planning operations -- a few sleeping bags on the floor, some food and little else.

Nonetheless, its closure and the arrest of Guridi and Goni set off celebrations in the Spanish government. The spokesman for Spain's ruling Popular Party called it "extraordinary news," and told journalists that the pair were "the bloodiest terrorists in ETA."

Special correspondent Caroline Huot contributed to this report.

Basque separatist military commanders Juan Antonio Olarra Guridi, left, and Ainhoa Mugica Goni used France as a base for attacks in Spain.