"IT WAS FIVE O'CLOCK on a winter's morning in Syria. Alongside the platform at Aleppo stood the train . . ." It isn't often remembered how Agatha Christie's most famous book begins, but it was Syria that put the "Orient" in Murder on the Orient Express, and Aleppo, the great highland city of the north, where her sleuth, Belgian detective Hercule Poirot, waited for his train. Christie was writing what she knew: Married to an archaeologist, she spent much of the 1930s living in a mud-brick house at a remote dig site outside Aleppo, typing furiously.

The UN had just sent its own Hercule Poirot into Syria to solve a case, namely, the assassination of Rafik Hariri in Beirut. The detective this time was Detlev Mehlis, a former prosecutor from Germany who, like Poirot on the Orient Express, had to sort through a rapidly expanding array of evidence that pointed him to the inescapable conclusion that, just like on the Orient Express, not only had almost everyone been involved but almost everyone was going to get away with it.

Who were the six men who shadowed the prime minister's convoy in Beirut? (They flew away 90 minutes after the bombing.) Who bought the six cell phones they used? (A Lebanese from a pro-Syria party.) Who drove the white Mitsubishi van into Lebanon? (A Syrian colonel.) Where was the van stored before being loaded with more than 2,200 pounds of TNT? (At a Syrian military base.) Which Syrian officials were present at the meeting last February where the crime was green-lighted? (The president's brother Maher and his brother-in-law, Asef Shawkat.)

For his own protection, Mehlis was stashed in a "secret location" outside Damascus—a red-roofed hospitality complex in the first range of hills, clearly visible from any Gran Fury crossing the border. Not long after he left, one of the suspects he'd interrogated, Interior Minister Ghazi Kanaan, blew his own brains out. (Or was it murder?) Now the case had everything, even a smoking gun.

Like Poirot, I greeted the dawn on a train platform in Aleppo, having made the early run up from Latakia. The train was less Orient Express than early Brezhnev, and the Aleppo station had been improved with murals of Baathist astronauts and victorious Syrian armies. Despite the murals, Aleppo has changed at a glacial pace over the past 6,000 years. Recent big events have included a sacking by the Tartars, in 1260. They pulled down the glorious 11th-century hammam—marauders disdain bathing—but it was rebuilt, and a scalding trip through the steam room, a vicious pummeling, and a cold orange soda set me back $5.

I wandered Aleppo's enormous Armenian quarter, packed with ancient churches and vendors of delicate sweets. (They also have the head of John the Baptist's father, in case you didn't get your fill in Damascus.) There were miraculous and medieval sights around every corner—soccer-playing imams in one place and, down the next alley, cheery children hammering metal inside a Dickensian workshop. Like any police state, Syria has little crime, so the only danger I faced in the souk, the huge covered marketplace, was being run down by a donkey messenger. Crumbling but alive, too poor to be ruined by progress, decorated with old cars and European tourists, Aleppo is a desert flower that persists only in the adverse conditions of geopolitical hostility and a moribund dictatorship.

In the 1950s and '60s, a misguided attempt at modernization bulldozed some wide avenues through these old neighborhoods to make way for a few featureless plazas and office towers. Only a nitpicker of satanic proportions would obsess on how these few alterations had ruined the city, but there was such a man: Mohammed Atta, the ringleader of the September 11 attacks. A student of urban planning in Hamburg, Atta had come to Aleppo to write his college thesis. The young Egyptian loathed the way that a Le Corbusier grid had been superimposed on the crazed streets of Aleppo, which he viewed as a pure and organic expression of Islamic social relations. He left in 1994, nurturing a seed of hatred for Western architecture.

I poured gin and tonic on Atta's soul in the bar at Baron's Hotel, a symbol of everything he hated—grandly European, frankly colonial, a faded headquarters for Western cultural subversion. Christie lodged here in the 1930s; on the wall was a bill from April 1, 1914, made out to "Monsieur Laurence":

4 jour pension 32 piastres
menu a repas 3 piastres
limonato 2 piastres

T.E. had walked much of the distance I'd just covered, arriving in Aleppo to discover himself reported dead in a local newspaper. He got over it: "Aleppo is all compact of colour, and sense of line," he wrote in a letter home. "You inhale Orient in lungloads, and glut your appetite with silks and dyed fantasies of clothes." The days were easy to fill as he had, roaming widely in the surrounding hills, consuming an endless buffet of ruins, civilizations piled atop one another. At the Dead Cities, a whole archipelago of old Roman towns, I met shepherd boys with the names of Christian emperors, and a Dutch archaeologist in a thrilled state of despair. Like Lawrence, like Atta, she was here for her thesis, trying to do six months of research on a one-month visa. Snooping where I shouldn't have, in fading light, I slipped and slid into a dank tomb. It was filled with cobwebs, a broken sarcophagus, and 1,500-year-old bones.

I shot out of that hole so fast, the shepherds were still laughing ten minutes later.