On May 30 1980, she and six friends, five boys and another girl, in wigs and sunglasses and with mopeds for getaway vehicles, raided a bank. A quarter of a century later, Hélène Castel, now 46, stood in a Paris courtroom yesterday to answer charges of armed robbery and hostage-taking.

She came very, very close to getting away with it: the police came knocking on her door in Mexico, where she was living and working as a psychotherapist named Florencia Rivera Martin, on May 12 last year. Had they waited four more days, she would have been safe under the statute of limitations.

Looking for the most part calm, with short dark hair and wearing grey trousers and a purple shawl, Ms Castel admitted taking part in the robbery of the rue Lafayette branch of the BNP bank nearly 26 years ago. It had been the great "débàcle" of her life, she told the court.

In a well-planned if naive operation, the seven youngsters rushed into the bank, one brandishing a sawn-off shotgun. Each had an assigned task: cut the phones, jam the alarms, snatch the notes from behind the counter, force the manager to open the safe. Ms Castel's job was to stand guard over the 37 staff.

It all went disastrously wrong. Two police cars were waiting outside when the robbers emerged; shots were exchanged. One of the men, Lionel Lemare, was hit and died on the spot and the bank manager, taken hostage, was badly wounded (although it was never clear by whom).

The last anyone saw of Hélène Castel, she was sprinting down a side-street of the rue des Martyrs behind the bank. The moped on which she was riding pillion was struggling to make it up the hill, and moments before he was arrested, the driver had urged her to get off and run.

Four alleged accomplices were caught and tried. One was acquitted, and three were sentenced to jail terms of five, eight and 10 years. Now a doctor, a historian and a craftsman, they rejoined society and are considered under French law as rehabilitated - which means they cannot in principle be named in any connection whatsoever with their past crimes.

As for Ms Castel, she went to ground, cut her hair, dyed it blonde, got hold of false ID papers and made it first to the US, where she spent three years, and then Mexico, where she survived as a house painter and silk-screen printer before training as a psychotherapist and setting up a practice in the town of Jalapa.

She was picked up without resistance by the Mexican police, under pressure from France's new Central Office for Missing Persons and Fugitives, set up by the interior minister, Nicolas Sarkozy, three months previously in February 2004.

She has a daughter Maria, who is the same age as her mother was in May 1980, and who will give evidence as to the law-abiding and "exemplary" life Ms Castel has led since she fled France. Her father Robert, a respected sociologist, is also due to give evidence.

Asked yesterday by the judge, Dominique Coujard, to describe her background, Ms Castel said she had a "solitary" childhood because her parents were "too taken up with their intellectual and social activity". At 20 she had quit her studies and left home, drawn by the squatters' "solidarity". When their squat was pulled down, they "needed money to build something new". She and her friends were "disillusioned, lost, in search of meaning", she said. "We weren't fully formed. That's why we identified ourselves with a lost cause."

Ms Castel was sentenced in absentia on May 16 1984 to life imprisonment, the automatic sanction for the crimes of which she had been convicted given that she was on the run. If convicted anew, she faces up to 20 years in jail.

Ms Castel told the court she had never expected she would be arrested because "no one seemed to be trying to find me". While she had often thought of turning herself in over the past 20 years, she had never done so because of her daughter.

"I wanted to wait until she was fully independent before taking that decision," she said. "When I was arrested, it was like a cold shower in a heatwave. I realised I really needed that, I needed to be reborn in France, to be whole once more."

She had been devastated when she learned of the consequences of the robbery, and deeply regretted taking part.

Her lawyer, Henri Leclerc, said the trial, which ends tomorrow, might allow her to find a solution to a life lived as a fugitive, even if "it's an absurdity: we are trying crimes committed 25 years ago and a person who is no longer who she was at the time."