A day after a terror strike rocked Mumbai, Prime Minister Manmohan Singh said the perpetrators must be 'pursued relentlessly' as investigators scrambled for clues on a rain-soaked Thursday and a weary metropolis picked up the pieces.

Eighteen people were killed and 131 others injured in synchronised blasts that rocked India's financial capital, striking the congested areas of Dadar, Zaveri Bazar and Opera House within minutes of each other on Wednesday evening - but a day after, there was no breakthrough on who was behind it.

'Terrorists surprised us'

Manmohan Singh, who visited the metropolis on Thursday evening with Congress president Sonia Gandhi, condemned the 'barbaric' bomb blasts, promising 'the government will do everything in its power to prevent such attacks in future'.

He said the government was coordinating 'efforts and resources to relentlessly pursue the perpetrators... (who) have sought to subvert'.

About the attack, the Prime Minister said 'the terrorists had the advantage of surprise' and that the 'administration in Maharashtra has risen remarkably well on this occasion'.

The Prime Minister and Sonia Gandhi also visited some of the injured in J.J. Hospital. They spent around 25 minutes at Saifee Hospital in central Mumbai where 37 of the 131 wounded are undergoing treatment.

They spoke with the patients, most of whom had suffered burn injuries, and wished them speedy recovery, besides inquiring about their condition from the doctors.

The investigation into Black Wednesday

Meanwhile, Maharashtra Anti-Terrorism Squad chief Rakesh Maria vowed that they would 'ensure no matter wherever the accused are, we will identify them and bring them to book' as he appealed for faith and trust.

Officials refused to name any particular group suspected to be behind the blasts for which there has so far been no claim of responsibility. But they said they were looking at 'all angles' to identify the perpetrators.

But strong suspicion has fallen on the Indian Mujahideen, a shadowy home-grown militant group known for its city-to-city bombing campaigns using small explosive devices planted in restaurants, at bus stops and on busy streets.

Various agencies, including the National Security Guard (NSG), the Central Forensic Science Laboratory (CFSL), the National Investigation Agency (NIA) and Mumbai Police, are involved in the probe.

As the investigators began searching for clues, the only thing that was sure was union Home Secretary R.K. Singh saying the ammonium nitrate-based IEDs were 'not crude' but indicated 'some level of sophistication'.

Home Minister P. Chidambaram confirmed the nature of the explosive saying: "Ammonium nitrate was used with a timer. The fact that they all took place within minutes of each other - eight-to-10 minutes - shows that it was a coordinated terror attack."

Sources said traces of ammonium nitrate, commonly used in fertilizers, and fuel had been found in the explosives.

A top doctor at one of the hospitals where the bodies of the dead were taken for a post-mortem examination said an electric circuit, that may have triggered the blast, was found on one of them, leading to speculation that it could have been a suicide bomber at work.

CCTV footage link

The CCTV footage holds the key. Sources said that some of the footage was marred by the rain and bad light on Wednesday evening.

"The volume (of footage) is huge and we are going through each and every footage."

The statements of the injured witnesses were also being recorded which would help police to get a clear picture about what had happened last evening, Maria added. Sources have added that agencies are also questioning certain Bangladeshi nationals, who have had dubious records in Surat, Gujrat.

He, however, declined to comment when asked whether the explosives were detonated through cellphones.

'Life limps back to normal'

Resigned, outraged or simply stoic, Mumbaikars rallied around to battle the crisis, just 31 months after the November 26-29, 2008 terror assault, India's worst. They waited outside morgues to claim the bodies of their kin, lined up at hospitals or packed into trains and buses to go about their daily work.

"I am a Mumbaikar and we shall not be scared by these terror attacks. Like me, lakhs of commuters are in the trains, buses and roads. It actually helps give strength to each other," Archana Shukla said as she went to work.

In Zaveri Bazar, Mumbai's most popular address for jewellery that Wednesday saw its third terror strike, merchants were shocked. But they firmly said they were staying put.

"What is the point in shifting base? Are other business locations safer?" asked Raju Solanki, a gold jeweller.

Why did the attacks happen now?

The timing of the attack has raised suspicions in informed strategic circles over whether the serial blasts were engineered by those who wanted to derail the peace process in the subcontinent.

Pakistan's foreign minister comes to New Delhi for talks on July 26-27 and External Affairs Minister S.M. Krishna said the terror strikes would not impact the strategic dialogue.

US Secretary of State Hillary Clinton touches down here on July 18 on a three-day visit that also includes a trip to Chennai, the hub of top-billing American investments.

Clinton will hold the second India-US strategic dialogue with External Affairs Minister S.M. Krishna here on Tuesday that will encompass a broad spectrum of issues ranging from counter-terrorism and security to civil nuclear cooperation, defence and closer cooperation in science and technology.