The Mosel was under Captain Leist that morning; her assigned master, Hermann Neynaber, had fallen ill. She had been towed by the tug Simson and lay temporarily alongside to finish loading. Among the freight was a barrel armoured in iron, unusually heavy, declared as caviar worth three thousand marks. When it fell, the blast tore a hole in the ship's side and blew a crater four metres deep in the quay. It was heard for miles. A black cloud went up over the harbour, and the people who were not killed outright lay on the stones screaming. The dead were dockworkers doing the loading, crew, passengers, and the families who had travelled to Bremerhaven for the sole purpose of saying goodbye to somebody they did not expect to see again — parents, grandparents, children, waiting on a pier to wave at a departing ship.

The barrel held roughly 650 kilograms of Lithofracteur, a commercial dynamite made by Gebrüder Krebs & Cie. Inside it sat a clock, commissioned from a clockmaker in Bernburg named Johann Ignaz Fuchs and built to run silently for ten days before releasing a hammer. The intention was that the Mosel would be far out in the Atlantic when the hammer fell: ship, cargo and everyone aboard on the sea floor, and an insurance claim on freight that no adjuster could ever examine. Alfred Nobel had patented dynamite only eight years earlier. Four hundred passengers were simply part of the arithmetic. What defeated the plan was the crane.

Six hours later, at five in the afternoon, someone opened a first-class cabin and found a passenger on the list as William King Thomas, covered in blood with a pistol beside him. He had shot himself twice in the head and was alive, paralysed. Two notes lay nearby: one bidding farewell to his wife and children, one addressed to the captain, regretting what he had done. In brief periods of consciousness he admitted that the barrel had been his. He was Alexander Keith Jr., born in Caithness in Scotland in 1827, nephew and namesake of the Halifax brewer. During the American Civil War he had run the blockade and carried messages for Confederate agents out of Halifax, tangled in the aftermath of the Chesapeake affair and in Luke Blackburn's scheme to send yellow-fever-infected clothing into northern cities. In 1865 he swindled his own associates and fled to St. Louis, and by the 1870s he was living in Germany under an assumed name. He died on 16 December at half past four in the afternoon.

Many of the bodies could not be recognised. Of the victims, only twenty-two were ever identified. On 14 December, three days after the explosion, forty-one of Bremerhaven's dead were buried together at the Wulsdorf cemetery, where the mass grave remains — which means that a great many families in that town were never told which coffin held their own. Others among the injured died over the following days. Keith's corpse, by contrast, was treated as material. A Bremerhaven doctor named Soldan severed the head and preserved it in a jar of alcohol, ostensibly for research; it was placed in Bremen's crime museum in 1914 and destroyed during the Second World War.

The Pinkerton agency was hired by the German authorities to unpick the fraud, and the report it produced traced Keith's earlier attempts — Bremerhaven had not been his first idea, only his most successful one. That report later became the backbone of Ann Larabee's 2005 book The Dynamite Fiend. The Mosel herself was repaired and went back to sea, sailing another seven years before she was wrecked off the Lizard in Cornwall on 9 August 1882. The thing that lasted longest was neither the ship nor the man but the method: a timer, a commercial explosive and an unremarkable parcel among ordinary freight, aimed at people who had no part in the quarrel and no idea they were in one. Germans still call it the Thomas-Katastrophe, after the name he was travelling under.