Iran's transport minister says the blast on a railway line that killed nearly 300 people was probably caused by human error.
Investigators believe an unhappy worker may have been responsible
Ahmad Khoram told an Iranian news agency the authorities had ruled out natural causes, such as an earthquake, and politically-motivated sabotage.
He said they are also investigating whether an unhappy worker let 51 wagons run away at Neyshabur, north-east Iran.
The train was carrying hazardous substances, which set alight.
"We have come to the conclusion that an error was committed," Mr Khoram was quoted by the student news agency ISNA as saying.
"On the basis of questioning people and in view of the existing evidence... the most likely cause was oversights and shortcomings on the part of railway personnel."
He said they had ruled out sabotage, as well as other factors such as an earthquake or wind.
Other possibilities - such as a deliberate action by a disgruntled employee - were being examined, he was quoted as saying.
The train carriages ran away in the early hours last week and derailed at Khayyam station, near Neyshabur, some 75 kilometres (50 miles) from the north-east city of Mashhad.
The cargo of sulphur, fertiliser, petrol and cotton set alight.
As firefighters doused the flames, local government officials and curious villagers gathered around to watch. Without warning, the cargo exploded.
The blast - described by Mr Khoram as being equivalent to 180 tons of TNT - devastated nearby villages.
Local news agencies put the death toll at around 289 dead - some 150 being firefighters and government officials - and 460 people injured.
Mr Khoram was also quoted as saying that, although none of the materials carried by the train were classified as dangerous, the disaster might lead to re-classification of some cargo.