"The six are believed to have connected with and know about the bomb explosion," National Police head of public relations division Inspector General Anton Bachrul Alam said here on Wednesday.
He said the police initially arrested 13 witnesses who submitted the body of the blast victim, Firdaus.
Later the police released seven of them and held the six others who are believed to know about the incident.
He said the police had been able to enter the location through repressive actions after its access had been blocked by the people.
"The police could enter the location following a meeting with the regional leadership with the help of officials of the West Nusa Tenggara administration, clerics and community leaders," he said.
Anton said the police confiscated several bomb disposals at the school but found no firearms.
He said police also did not find students in the location who were believed to have fled to the mountain.
The bomb blast at around 3.30pm on Monday (July 11) killed a teacher of the school, Firdaus.