While two bombs in one coach did not explode due to a “timing” error, four bombs went off in each of the other two coaches killing 68 people. Only 12 passengers of the two coaches survived.

Police recovered the two suitcase bombs that did not explode. While one was lying on the railway tracks, the other was found on the first berth next to the door.

Before it was detonated by the police, the first clear fingerprints were lifted from seals of the 14 plastic bottles found inside the suitcase. The Haryana Police have sent the fingerprints to other state police units and intelligence agencies.

“All the bombs were to go off at the same time — 12 midnight. But in the two bombs that did not explode, the timer was set at 12:00 instead of 00:00,” said SP Railways (Ambala) Bharti Arora, part of the SIT formed by Haryana Police to probe the blasts.

Dr R K Kaushal, assistant director of the Haryana Forensic Science Laboratory in Madhuban who is leading the investigations, said while some bottles contained kerosene, others had petrol or a mix of both. “The seals were probably put to cover any smell. It also helped the explosives to go off with maximum pressure,” he said.

Stating that the bombs were “made and planted with great precision”, Dr Kaushal added: “Studying the spread of fire and other clues in the two coaches, it is clear that the fire spread towards the centre. Clearly, the motive was to explode the bombs at either end to jam the doors, so that there was no escape route left.” While all four doors of one coach could not open, only one door of the other coach could be opened.

Forensic officials said the bombs went off at the set time after sparking in the electronic “light-emitting diodes” that were part of the connecting circuit. The sparking led to an explosion in the pipes containing potassium nitrate with sulphur, along with carbon and magnesium wires. This then spread to the kerosene-petrol bottles.