RDX was used in the Shingar cinema hall blast in Ludhiana that left six dead and 30 wounded, the Punjab police said on Friday.

"Traces of RDX have been found. The quantity of RDX and other explosives used in the blast is half-a-kg or less," ADGP (Law and Order) Chandershekhar told PTI.

Forensic, NSG and army experts had confirmed the use of the lethal explosive in the blast, he said.

However, the intensity of the blast was not very high, he said, quoting the team probing the case.

The confirmation of the RDX had established involvement of cross-border anti-national forces in the incident, the ADGP said.

He said police was zeroing in on the suspects, adding that investigations were in full swing and it was not desirable to give further details at this stage.

A bomb blast in the cinema hall in Punjab's industrial town on Sunday had killed six people and left 30 injured, mostly migrant labourers.

A report from Ludhiana said investigators probing the blast had so far questioned 60 people, including former terrorists and their family members.