Indian investigators looking into Wednesday's blasts in Mumbai have been examining security camera footage for vital clues, media reports say.
An unnamed detective told the state-run DD News that footage from one of the sites revealed "suspicious behaviour of some people"..
The three explosions killed 18 people and injured dozens.
The attacks are the deadliest in India since 2008, when gunmen killed 165 people in a three-day raid in Mumbai.
Home Minister P Chidambaram has said the blasts could have been the work of a "small group" who had "worked in a very clandestine manner".
No group has said it planted the bombs but suspicion among some officials and analysts has fallen on the Indian Mujahideen, a group which has claimed to have carried out similar attacks in the past.
Investigators who are going through the security camera footage and forensic evidence from the three sites are hoping to get a breakthrough, media reports say.
The biggest explosion occurred at the Opera House business district in the south of the city, in an area known as a hub for diamond traders. The detective quoted by DD News said footage from the site showed people behaving suspiciously.
"But until we question them or record their statement, we cannot say anything about their involvement," he added.
"We are calling all those people seen in the footage to get a clear picture of the incident or any details that may help us in the probe."
On Thursday Prime Minister Manmohan Singh said the "perpetrators of [the] Mumbai blasts shall be pursued relentlessly and brought to justice quickly".
The other blasts hit Zaveri Bazaar, an area with many jewellery shops, and Dadar district in the city centre.
Mumbai has been targeted many times in recent years.
The 2008 attacks, which focused on two high-end hotels, a busy train station, a Jewish centre and other sites frequented by foreigners, were blamed on the Pakistan-based Lashkar-e-Taiba militant group.
The gunmen, nine of whom died in the raid, killed 165 people.
Pakistan-based militants were blamed for the November 2008 attacks and peace efforts between the two countries were derailed. Pakistan's government was quick to condemn the latest bombings.