Indian investigators were today examining forensic evidence and CCTV footage for clues about who orchestrated the three bombings that killed 17 people in Mumbai.
Police have refused to speculate on who might have been behind Wednesday's blasts - in contrast to the 2008 siege in Mumbai, when India swiftly accused Pakistan-based Islamist militants after capturing one of the gunmen.
With no such leads in this week's attack, police are leading a painstaking investigation, combing through 11 CDs of surveillance video from the sites of the explosions, looking for suspicious people or activity, India's Home Secretary RK Singh told reporters in New Delhi.
Investigators were also studying forensic evidence collected in the three crowded neighbourhoods that were attacked during rush hour, Prithviraj Chavan, the top elected official of Maharashtra state where Mumbai is located, told CNN-IBN news channel.
The attackers placed one bomb on a bus shelter, hid another under some garbage on the road and stashed the third under an umbrella, officials said. All were improvised explosive devices made of ammonium nitrate with electronic detonators, authorities said.
The bombings were the worst terrorist attack in Mumbai since a 2008 siege in which 166 people were killed over three days.
No one has claimed responsibility for Wednesday's attacks and officials would not speculate on who might be to blame. India is battling dozens of rebel groups and insurgencies across the country.
Dozens of separatist rebel groups are active in the Muslim-majority Himalayan region of India-controlled Kashmir fighting for independence or merger with neighbouring Pakistan. In the remote forests of the north east, dozens of rebel groups are fighting for autonomy or independence. Maoist rebels have waged a bloody battle in several Indian states for more than four decades, demanding land and jobs for landless farmers and the poor.
Home Minister Palaniappan Chidambaram said yesterday that every city in India was vulnerable to terrorist attacks by virtue of living "in the most troubled neighbourhood in the world" near Pakistan and Afghanistan.
In the past, Indian officials have accused Pakistan's powerful spy agency of helping to coordinate and fund earlier attacks, including the 2008 Mumbai attack. Peace talks between the countries were suspended after that attack and resumed only recently.
Mr Chidambaram did not rule out that the blasts might have been aimed at derailing a new round of talks between the two nations' foreign ministers expected to start in two weeks.
Investigators often make initial arrests fairly quickly after attacks like the one that shook Mumbai on Wednesday, but detailed investigations are often sluggish and actual convictions even slower.
Indian authorities are still investigating a 2007 train bombing that killed 68 people. The trial in a 1993 serial bombing in Mumbai that killed 257 people - the country's deadliest terrorist attacks - took 14 years.
Mumbai, a city of 18 million people, is the heart of India's business community. It houses the country's stock exchange and the popular Bollywood film industry.