The international commission investigating the murder of the former Lebanese prime minister Rafiq Hariri was widely expected to take one look at the challenge it faced and throw its hands up in despair. But Detlev Mehlis, the German state prosecutor in charge, had other plans. He likes to get his man. So, over the past six months, observers across the Middle East have watched with a mixture of elation, fear and astonishment as he has worked his way higher and higher up the power structure of Lebanese politics in his quest for the truth about who was behind the bomb that killed Hariri. At the end of August he initiated the arrest of four senior security figures in Lebanon, an event unthinkable just a year ago. Now his report on the case is due and the big question is: will he point the finger of blame at Syria, which is widely believed to be behind the murder?

Mehlis has dropped no hints and not even UN Security Council members will know what conclusion he has reached until he releases his report to them. Meanwhile, as he sat in Vienna putting the final touches to a report that will influence the fate of the whole region, from Syria and Lebanon to Israel, Egypt and Saudi Arabia, important people were scrambling into diplomatic crash positions.

Superficially, the crime he has investigated was hardly a mystery story. Hariri was a powerful figurehead for growing frustration with the excesses of Lebanon's Syrian occupiers. He had resigned as prime minister in October 2004 and was believed to be contemplating a return to power on an anti-Syrian platform. When his life was ended in such emphatic fashion on 14 February this year, the culprit seemed almost too obvious. In the words of one of the arrested security chiefs, Jamil al-Sayyed, this murder was planned "either by an Einstein or by a donkey".

Most Lebanese favour the donkey theory. Hariri's political allies, quick to capitalise on outrage at the assassination, plastered Saatchi & Saatchi-designed "independence" posters all over Beirut, along with the slogan "al-haqiqa" - "the truth".

They were not the only ones to harness the case to political ends. The United States, frustrated at Syria's perceived support of insurgents in Iraq, was keen for ammunition against Damascus, while Israel wanted to see the militant groups associated with Syria, such as Hezbollah, weakened. But the Syrian regime of Bashar al-Assad yielded as little as possible. Although popular demonstrations and international pressure forced it to begin withdrawing its troops from Lebanon in March, a recent murderous spate of attacks on anti-Syrian figures in Lebanon sent a clear message to anyone who thought the tide had turned against the old regional power.

This was the venomously charged background against which Mehlis had to carry out his investigation. His temperament and record made him well suited to the job. He was the brilliant prosecutor who successfully convicted Libyan agents involved in the bombing of La Belle, a Berlin nightclub, in 1986, and a series of high-profile international terrorism cases since then has helped him build on a reputation for forensic rigour and exactitude. Now, in Beirut, he has led a team of 100 investigators and forensic experts from more than 50 countries as they painstakingly searched through blasted rubble, seaweed, mobile-phone records and thousands of tonnes of documents for evidence of who killed Hariri, and how.

Colleagues speak of his intense focus, his modesty and of the respect he commands from his team. This last is just as well, because for six months they have lived, eaten and relaxed toge-ther, as well as worked, in the same heavily guarded hotel in Monteverde, just outside Beirut - chosen as the only place secure enough for one of the most targeted groups of people in the world.

Mehlis, who is 55, does not work from hunches a la Columbo; his method is to supervise and direct the collection of evidence. His determination, his thoroughness and his apparent indifference to the political games going on around him have won him praise in the Lebanese press. Though Mehlis once said in an interview that his investigation dealt in "shades of grey" his spokesman promised that the report would be "in black and white".

For all that he appears to be the quintessential European bureaucrat, Mehlis is clearly not without cunning or political instincts. During the La Belle investigation, according to a German documentary, he used his intelligence connections to meet one of the key suspects (and sources) for a beer in Malta, cutting a deal under which the suspect turned himself in to the German embassy the next day. Some of the same talents were presumably behind the coup he pulled off last month in negotiating access to senior Syrian officials in Damascus, though he is said to be unhappy with the conditions under which the interviews took place. It is said that he took a tactical decision not to go after Syrian-allied journalists in Beirut, and some have claimed that he had no hard evidence linking the four security chiefs to the murder and that their arrests were simply a tactical manoeuvre (although, as they remain in Lebanese custody, this seems unlikely). In his few press statements he has been keen to create a sense of the inevitability of the justice process, confidently announcing that the investigation was "winning".

Whatever else he may be, Detlev Mehlis is certainly his own man. He is not a UN employee, and reports only to the Lebanese government and UN Security Council. He showed his ability to stand up to pressure when Russia and Algeria demanded a preview of his report and he flatly refused. And crucially, per-haps, he has also shown he is prepared to break with the popular narrative of the anti-Syrian opposition in Lebanon, by refuting the theory that the bomb which killed Hariri was below ground. It was in a lorry.

As the investigation has ground onwards the Middle East rumour mill has gone into overdrive, offering wildly different answers. There have been suggestions that the beleaguered regime in Damascus will make a deal with the US, offering sacrificial figures to the investigation and co-operation on Iraq and Palestine, provided Assad himself is spared in the Mehlis report. This seems unlikely. As the Times journalist Nick Blanford, author of a forthcoming book on the assassination, explains, "It is difficult to see how such a deal could be reflected in Mehlis's report because his approach is so forensic and legalistic." Put simply, so far as the German prosecutor is concerned, either the evidence is there or it isn't.

Even if Assad himself is not implicated, as a recent article in the German press suggested, that would not necessarily save his regime. If Mehlis fingers Assad's brother Maher or his brother-in-law Asef Shawkat, both linked to the compromised Lebanese security apparatus, the Syrian president will be in a catch-22: if he hands them over to an investigation his regime has damned as politically motivated, he will lose so much credibility at home that he might even face an internal coup; if he doesn't hand them over, he courts international sanctions or worse.

Another rumour suggests that Mehlis will ask for a six-month extension of his mandate. He has already made clear that he does not have much confidence in the Lebanese government's ability to follow up the leads he has uncovered. In fact, after six months of Mehlis's probing, the Beirut government, which ironically it was Mehlis's mandate was to "assist", looks to be on the verge of collapse. Another possibility is that Mehlis will produce an inconclusive report. This would create a diplomatic victory for Syria and would probably result in a backlash against the relative openness and independence that have been allowed to develop in Lebanon since Hariri's assassination.

The regional powers are sufficiently alarmed at the prospect of a "smoking gun" report to be in full diplomatic damage-limitation mode. While Assad's regime has few fans, the chaos that might succeed it has even fewer, particularly with the rise of political Islam across the Middle East. The Egyptian president, Hosni Mubarak, met Assad late last month and his spokesman declared afterwards: "The stability of Syria is the stability of the region." Saudi Arabia and Egypt have had a series of meetings and Israel has told the US that it would prefer a weakened to a changed regime in Damascus. In Lebanon itself, normal life has come to a standstill. Weddings, business deals, house purchases - all have been put off until after Mehlis's report. The government announced special security measures ahead of the report's release.

Meanwhile, in Vienna, the methodical German was checking his work and preparing for the storm. He is said to be acutely conscious of the political ramifications of his report, but he plays down his own importance, insisting that he is only summarising the facts in the hope that he can "as much as possible let people draw their own conclusions". Seldom can a detective have carried so much responsibility.