The police detained as many as 36 people on Monday from Ludhiana and other places in Punjab as investigators searched for clues to the blast in a crowded cinema hall in this industrial city on Sunday evening.
The toll in the blast inside the Shingaar cinema hall went up to seven on Monday, the chief medical officer at Ludhiana's civil hospital said.
The explosion took place just before 9 pm when a Bhojpuri film - Janam Janam Ke Saath - was being screened and the movie had restarted after the interval.
About 30 people were injured, some of them seriously.
The Punjab police, security agencies and forensic experts tried to gather shreds of evidence about those behind the powerful blast.
Deputy Inspector General of Police (Intelligence) Jagdish Mittal said four police teams were constituted to investigate the blast and uncover the conspiracy behind it.
Mobile telephone calls were being traced in Ludhiana and other places to get clues, senior police officials said.
"We do not rule out the involvement of Babbar Khalsa and some Pakistan-sponsored terror outfits in the blast. The blast indicates the hand of Pakistani Inter-Services Intelligence (ISI) and it may have activated some sleeping terrorist cell to execute the blast. The Khalistan Commando Force, the Khalistan Zindabad Force and some Muslim jehadi groups could be behind the blast," Mittal said.
Police officials said it had become clear that those targeted were migrant labourers and workers from Uttar Pradesh, Bihar and other states.
All those killed, including a 10-year-old boy, and the majority of the injured were migrant workers who had settled for work in this city, one of the biggest industrial cities in Asia.
Those behind the blasts carefully targeted the theatre that was regularly showing Bhojpuri films for the migrant population. Being the festival of Eid and a Sunday, there was a huge rush at the theatre.
A one-third of the 3.5 million residents of Ludhiana and its nearby areas are migrant workers.
Chief Minister Parkash Singh Badal said terrorism would not be allowed to raise its head in Punjab again.
"We will ensure greater security and also bring the perpetrators of this crime to book. We will ensure full security of migrants in the state," Badal said in Ludhiana on Monday.
Punjab police officials investigating the blast said they were working on all theories, including a possible link to Kashmiri terrorist outfits and Babbar Khalsa International (BKI), a terrorist organisation of Khalistani separatists.
"A forensic team from the National Security Guards has also arrived. We are working on various theories," said Inspector General of Police (Jalandhar range) Sanjiv Kalra.
Former Punjab super-cop KPS Gill pointed out: "Cells of the BKI are trying to reactivate (themselves) in Punjab. They still have their links in Pakistan and also with other terrorist outfits. The cinema hall blast here came soon after the bomb blast at the Sufi shrine of Ajmer in Rajasthan Friday."
Security officials admitted they had no intelligence tip-off that Punjab could be a terror target, though the state's Director General of Police NPS Aulakh said: "We had issued an alert for the festival season."
The state was put on red alert after the blast.
The names of Punjab-based terrorist groups like the BKI did figure in the twin bomb blasts in cinema halls in Delhi in 2005. There have been reports occasionally with security agencies here that terrorist groups had made several attempts to regroup and reactive their cells in Punjab. The Punjab police seized over six kg of RDX in January this year near Jalandhar.
The police are probing all angles to the blast, including the recent conviction and awarding of death penalty to Babbar Khalsa terrorist Jagtar Singh Hawara and others in the assassination of former chief minister Beant Singh.