Mumbai/New Delhi, Jul 14 (IANS): The improvised explosive devices (IEDs) used in the triple bombings were not crude but prepared with "some level of sophistication", the government said as investigators scrambled for clues on a rain-soaked Thursday and a weary Mumbai picked up the pieces after yet another terror strike.

A day after 17 people were killed and 131 injured when synchronised blasts rocked India's financial capital, striking the congested areas of Dadar, Zaveri Bazaar and Opera House within minutes of each other, there was no breakthrough on who was behind it.

"We will ensure no matter wherever the accused are, we will identify them and bring them to book," Maharashtra Anti-Terrorism Squad (ATS) chief Rakesh Maria vowed, appealing for faith and trust.

In a flurry of activity, Prime Minister Manmohan Singh and Congress chief Sonia Gandhi headed for Mumbai. Home Minister P. Chidambaram reached Wednesday night and opposition leader L.K. Advani Thursday morning.

Resigned, outraged or simply stoic, Mumbaikars rallied around to battle the crisis, 31 months after the Nov 26-28, 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 co-commuters are in the trains, buses and roads. It actually helps gives 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 said firmly they were staying put.

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

That was a question even experts were loathe to answer as investigators began unravelling the conspiracy behind this latest assault by yet unknown terrorists.

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

According to union Home Secretary R.K. Singh, the ammonium nitrate-based IEDs were "not crude" but indicated "some level of sophistication".

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

Sources said traces of ammonium nitrate, also confirmed by Chidambaram, and fuel had been found in the explosives.

"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," Chidambaram said.

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

Hours after visiting the terror sites, Chidambaram said 131 people had been taken to 13 hospitals with injuries. He said one severed head was also found at the site that could take the death toll to 18.

Addressing a press conference in Mumbai, he admitted there was no prior intelligence input.

Quick to seize the initiative, BJP leader Advani pointed the needle of suspicion towards Pakistan.

"It is a policy failure not intelligence failure. There have been repeated attacks on Mumbai, this is a failure of policy," Advani told reporters.

Advani quoted reports of a probable link between the blast and the Indian Mujahideen (IM) and said even if it were behind the attack it was being sustained by Pakistan.

"The last attack on our land is proved to have been engineered by the ISI (Inter-Services Intelligence)," Advani said.

"So far as India is concerned, the government of India should shed its ambivalence to terrorism," Advani said.

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

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.