"Pretty Woman", as she was known in the distant days of the underground, speaks softly in the recorded message that plays to callers who telephone her home: "I have found out a lot about myself, about where I am now. When I meet myself I will call you back."

That final encounter has yet to take place. For 16 months, Monika Haas, divorced mother of three and allegedly one of the most dangerous German terrorists alive, has been separated from her disembodied voice by the thick walls of Frankfurt's top security prison. Last month, she was allowed out for one week, but still only as a captive, to receive urgent medical treatment under armed guard in a civilian hospital.

Yesterday, the Supreme Court in Karlsruhe ruled that, despite her illness, she must stand trial for her part in the cataclysmic events of the "German autumn" of 1977, an orgy of violence that climaxed in the storming of a hijacked Lufthansa plane in Mogadishu, the murder of the industrialist Hans-Martin Schleyer and the apparent suicides in prison of Andreas Baader and two other leaders of the Red Army Faction.

Most of the key players in the bloodiest episode in Germany's post-war history have since been killed or captured. But there has long been one figure missing from the cast list. The mysterious blonde who in October 1977 smuggled the grenades, guns and detonator in a pram to the hijackers in Majorca - where the Mogadishu plane began its flight - has never been caught.

Now, after nearly 20 years of searching, the state has come up with a name to match the description of that blonde. Monika Haas, former Red Army Faction sympathiser, one-time courier for a Palestinian commando unit, ex-wife of a Palestinian guerrilla leader, but latterly equal opportunities officer at Frankfurt University, was plucked out of her suburban bliss at the age of 46.

Whether she will ever be allowed to see her children again, beyond the allotted one hour a week, depends on the testimony of another mother, Souhaila Andrawes, the only Palestinian hijacker to have survived Mogadishu, who gave the world a V for victory sign as she was stretchered from the plane. Andrawes was caught in Norway in October 1994, separated from her nine-year-old daughter, and brought to Germany to face trial. She served a year and a half in a Somali jail, and the German authorities have made it clear that they are prepared to waive a longer sentence in exchange for one little favour: if she identifies the woman who is the missing link in the story, then she will soon be reunited with her daughter.

With her statement, the case against Haas seems watertight. Andrawes's testimony underpins incriminating files recovered from the Stasi, the former East German secret police, which cast Haas as the linchpin of the Red Army Faction's last-gasp offensive against the German state. She had the right track record, all the necessary connections, a secure base in communist South Yemen and the sort of supportive husband an urban guerrilla could only dream of.

Yet the German media remain unconvinced of her guilt. The newspapers, even those with a conservative tinge, have been unusually restrained in reporting the Haas case, giving the impression that they are beginning to smell a rat. Naturally, they are worried about Andrawes's evidence. Also, the Stasi documents produced by the prosecution were thrown out by a previous court in 1992, an episode in which Haas was held for five weeks and then released. But most importantly, Germans wonder why, if Monika Haas was such an important villain, it took police 12 years to catch up with her at the family's Frankfurt address. She and the children had returned to Frankfurt from South Yemen in 1982 after her marriage broke up: before re-entering West Germany she had checked whether there was any case against her outstanding and had been told that there was not.

The Haas home is not exactly hard to track down. Her name is listed in the telephone directory, the number now connecting to the enigmatic answering machine. The apartment, a modest three-bedroom affair, is situated in a turn-of-the century block in the city's Sachsenhausen district. It is a quiet neighbourhood of small cars; the kind of place where residents know each other by sight but never say hello. The flat on the first floor now only has two inhabitants. Frank Haas, the fruit of his mother's short- lived passion with an American GI in 1968, moved out a day before the police turned up unexpectedly to take Monika Haas away. Hanna, aged 17, and Felix, 15, have been fending for themselves ever since. A social worker plays surrogate mother for 16 hours a week. After paying the rent, the teenagers have between them DM650 (about pounds 300) a month to play with, courtesy of the welfare state.

Frank, who lives with his girlfriend across town, runs errands for the "kids" but is proud of the fact that they seem to manage quite well on their own. "It is important that they learn to be responsible for their lives," he says sternly, as he demonstrates the order reigning in the bedrooms.

It is no use looking for guns or Red Army Faction memorabilia - the police searched the apartment and naturally found nothing. The clues to Monika Haas's extraordinary life are spread out, like pieces of a fiendishly complex jigsaw puzzle, in the family album. There are pictures of Haas in South Yemen in the late Seventies, of her and the children on holiday in Lebanon in the early Eighties. Later images capture the struggle for normality in Frankfurt after her return from the Middle East in 1982: driving a taxi to pay the bills, attending evening classes to prepare for the Abitur, the German equivalent of A-levels.

Flicking backwards through the albums, the jagged time sequence presented by the snapshots comes to an abrupt stop around 1977. Even Frank seems at a loss to explain what his mother did before that. All that is known for certain is that in the early Seventies Monika Haas was caught up in the maelstrom of radical politics, and was close enough to the Red Army Faction leadership to earn their confidence. The faction was the tip of an iceberg of left-wing radicalism, its leadership built around the core of the Baader-Meinhof gang which began operating in 1968. By her own admission, Monika Haas was an Red Army Faction "sympathiser", active in organising the defence of jailed terrorists. So active, in fact, that when the panic- stricken authorities tried to cut the jailed terrorists' lines of communication, Haas became engaged to one of them in order to be able to visit him. The "fiance" was Werner Hoppe, a member of the Baader-Meinhof gang's inner circle, and it is safe to assume that the two did not discuss the weather during visiting hours. Hoppe has only recently regained his freedom, a shell of a man with a mind unhinged.

By 1975, the police had gained special powers to arrest Red Army Faction sympathisers, and Monika Haas decided it was time to go underground. She deposited Frank, then aged seven, with friends in Osnabruck and disappeared, re-emerging at a Palestinian training camp in the Middle East, where, by her own admission, she learnt the art of wielding a gun. There she also met Zaki Helou, the leader in Yemen of the Popular Front for the Liberation of Palestine, a revolutionary group with strong links to European radical movements. Helou was later to become her husband, the father of Hanna and Felix.

In 1976, Monika Haas was caught for the only time in her life. She was allegedly delivering an envelope containing instructions for a commando attack on an Israeli plane when Kenyan police intercepted her at Nairobi airport. Three days and three nights of gruelling interrogation followed, as the Kenyans threatened to put her "six feet under" unless she co-operated by revealing the identity of her Palestinian cohorts. Then they inexplicably let her go, in circumstances suspicious enough to foster the myth in the Arab world that she had been "turned" by the Israeli secret service, Mossad.

After the Kenyan ordeal, Haas claims she abandoned the struggle, fearful for her safety, embarking on what she later described as the "most middle- class period in my life". Frank was collected from Germany and brought to the new family home in Aden, South Yemen, to join Zaki Helou. "She told me I was going to have a father," Frank recalls. "I was awoken at the airport by a man kissing my forehead. I looked up and thought: `Oh, what an old man.' He was 40. My mother's friends in Germany were all very young."

Zaki Helou was well paid by his employers. Frank went to Aden's International School, a private establishment with a distinctly British flavour, caning included. He learnt to play cricket on the school's dusty yard and entertained himself with comics flown in from England. "For me as a child it was a lovely place," he says. "We read all the Famous Fives and Biggles there." In the balmy evenings, the family would drive down from their home in the diplomatic quarter to the Gold Mohur Beach Club, sometimes without the stepfather, who was away a lot on business.

Frank is certain that he spent 17 October 1977 with both his parents. That was the day that the Lufthansa jet, commandeered by four Palestinians demanding the release of their comrades and Red Army Faction prisoners in Germany, called in at Aden on the penultimate stop of its odyssey. It was there that the hijackers shot the pilot in the head and dumped his body on the tarmac. "By chance, I was on the roof of the house and I saw it land," Frank recalls. "And I remember that evening when we had our ride my mother and stepfather took me there. My mother was definitely there on that day."

The "alibi" furnished by Frank is not enough to get his mother off the hook. Andrawes says the weapons, hidden in sweet tins in the pram under a three-month-old baby, were handed over on 7 October, giving the courier ample time to escape. But the description of the infant seems important. Was Andrawes "coached" by her German interrogators when she hit upon the exact age of Monika Haas's daughter Hanna, born in July 1977? The defence, in any case, has medical records to prove that Hanna nearly died of dysentery around the age of two to three months - and therefore was in no fit condition to make the round trip to Majorca.

In the end, that evidence is still only circumstantial. In pronouncing its verdict, the court will have to weigh the relative merits of the two testimonies: the word of a convicted terrorist against the word of a terrorist suspect. But again, as in the Andrawes case, the outcome seems irrelevant. What the prosecution hopes to gain from the trial is not justice, but yet another damning testimony, against a third woman.

"My mother is the central domino," Frank explains. The official screenplay has cast her as crown witness in the trial of Brigitte Monhaupt, a Red Army Faction member discovered hiding in East Germany when the Wall and her Stasi protectors melted away. Monhaupt was promptly imprisoned, but not for the crime she is suspected of: the garotting of Hans-Martin Schleyer, whose body was discovered in a car boot in Alsace the day after the raid at Mogadishu. German prosecutors are intent on proving that Monhaupt was guilty of that killing. Now Haas, denounced by a former comrade, is being given the chance to pass on the treachery by denouncing Monhaupt.

But the authorities may not have bargained for the resilience of Monika Haas. Already, in the 16 months she has been held, she has scored a minor victory by successfully suing Frankfurt University, which had attempted to dismiss her because of her non-attendance. And the chances of her denouncing Monhaupt look slight: while she has turned her back on violence, her politics have not altered. "She is tough enough," Frank Haas says. "She has no regrets about her politics and her conviction gives her strength."

Thus are the destinies of three women of the "German autumn" welded together, more than a decade after the defeated Red Army Faction renounced violence. Whether motivated by the urge for vengeance or merely bureaucratic meticulousness, the authorities seem determined to tidy up the files, at whatever human cost. "The [Red Army Faction] really embarrassed the authorities because they were the only group that put the state on the defensive," Frank Haas says. "It seems as though the pride of the judiciary was hurt. Now they are sending out a message: that no one will escape."