A crude bomb was used to trigger the explosion at the McDonald's outlet at Mumbai Central station on December 6, a report submitted by forensic experts to the railway police said on Monday.

"The report has confirmed the use of gunpowder and iron nails in preparation of the crude bomb," a top railway police officer said.

"The report, which was submitted at around 1830 IST, has confirmed our doubts about the use of a crude bomb in the explosion that injured 24 persons," the officer said.

Earlier, there was speculation that the explosion was caused by a faulty air-conditioning duct in the food plaza. The police, however, modified its statement to include the possibility of a crude bomb after doctors recovered nails and splinters from the victims, who also suffered serious burns.

The food plaza also issued a statement clarifying that its air-conditioning unit, its gas cylinders, and other equipment were all intact, indicating that the blast had been set off through some other device.