Indonesian prosecutors have again rejected a criminal case against those allegedly responsible for a disastrous "mud volcano" which has forced thousands to flee their homes, reports said Sunday.

Prosecutors returned the case to police citing a lack of evidence, the fourth time they have knocked back the brief since police started their investigations into the 2006 cause of the volcano.

"We have returned the dossier to the police for the fourth time," East Java prosecutor Mulyono told state-run news agency Antara.

The volcano has been spewing hot sludge -- as much as 53 Olympic swimming pools a day -- in Sidoarjo district in East Java since May 2006, swamping villages and forcing almost 36,000 people to evacuate.

Police in East Java have struggled to put together a dossier of evidence to indict 13 mid-level executives and field workers from oil and gas firm Lapindo Brantas and subcontractors for negligence over the disaster.

Mulyono said the main reason for the lack of progress in the case was the ongoing debate over what triggered the volcano.

"Some experts call it a natural disaster and the others said Lapindo is the cause," he said, adding that such differences about the cause would weaken any case that proceeded to trial.

A recent study by foreign scientists found the volcano was caused by bungled gas drilling by Brantas, not an earthquake as argued by the company.

Police have said they would consider using the results of the study, which were published in an academic journal, to strengthen their case.