By Omer Farooq BBC reporter in Hyderabad

Police in the southern Indian state of Andhra Pradesh have opened a criminal investigation into the derailment of an express train last Saturday.

At least 20 people were killed and more than 80 others injured in the incident, in the state's Kurnool district.

The police investigation comes a day after forensic scientists confirmed sabotage as the cause of the derailment.

The authorities have also sought the assistance of a defence research laboratory to establish whether the rail was deliberately cut.

Two teams of detectives from the Criminal Investigation Department (CID) have been sent to the derailment site in Kurnool.

Officials said the detectives were expected to investigate all possible angles.

Rail 'cut'

The state government has also sought the help of experts from the Hyderabad-based Defence Metallurgical Research Laboratory to establish how the rail was cut.

The pieces of the rail from the spot have been sent to the laboratory.

Initially poor track repair was thought a likely cause

Officials said scientists have been asked to ascertain the quality of the rail, among other things.

The case was referred to the CID and the defence experts after the state's Chief Minister, Chandrababu Naidu, expressed dissatisfaction over the findings of forensic scientists.

They had said that the rail was deliberately cut.

Mr Naidu was also said to have been unhappy that the police and forensic department went public with the findings, alleging sabotage before investigations were complete.

Mr Naidu is said to have reviewed the case at a meeting of high-level police officials.

The chief minister later told journalists that it was premature to describe the incident as sabotage.