The triple bombing that killed 17 in the heart of India's financial capital sparked anger Thursday over the government's inability to prevent deadly attacks despite overhauling security forces after the 2008 Mumbai siege.

Indian officials say they have made extraordinary security reforms since 10 Pakistanis rampaged across the city nearly three years ago, but following Wednesday's attack they warned they may never be able to guarantee a terror-free nation in a region plagued by extremism.

"We live in the most troubled neighbourhood in the world," said Indian Home Minister Palaniappan Chidambaram, pointing to nearby Pakistan and Afghanistan. "Every part of India is vulnerable."

No group claimed responsibility — and investigators had no immediate suspects — in the bombings that shook three separate neighbourhoods within minutes during Wednesday's busy evening rush.

Why India?

India has been wracked by attacks by a variety of assailants since gaining independence from Britain in 1947.

Its population of 1.2 billion is comprised of numerous — and in some cases competing — ethnicities, and the divide between the rising middle class and those still mired deep in poverty has added to tensions.

The South Asia Terrorism Portal, which tracks attacks in the region, lists more than 170 terror, extremist or insurgent groups in the country.

India has also fought three wars with archrival Pakistan and accuses its neighbour of actively supporting terror attacks by Pakistani-based militants on Indian soil.

Though India made significant investments in its security forces after the 2008 Mumbai attacks, its police forces remain poorly trained, undersupplied and tainted by corruption.

Why Mumbai?

Mumbai, India's commercial and entertainment hub, is crowded with 18 million people, billionaire businessmen and Bollywood superstars and has repeatedly been targeted by groups seeking to reap maximum exposure from their attacks.

The 2008 attacks by 10 Pakistan-based militants targeted India's busiest train station, a Jewish center and two luxury hotels in the city and left 166 people dead.

Since then, the city had escaped further attack, until the three coordinated bombings Wednesday night, which killed 17 people.

A bombing in Mumbai get noticed internationally. Its density likely means more victims, a changing population makes policing difficult,  and high migration numbers and a diverse population means it's easier for the attackers to operate.

Chidambaram said the government had no intelligence warning. "Whoever has perpetrated this attack has worked in a very, very clandestine manner," he said.

Prime Minister Manmohan Singh, who flew to Mumbai to meet with the victims, called on authorities "to relentlessly pursue the perpetrators. They must be brought to justice quickly."

"I assure the people that the government will do everything in its power to prevent such attacks in the future," he said.

But many remained frustrated.

"Why is Mumbai being attacked again?" said Uttam Jain, who works in a gold shop in the Jhaveri Bazaar jewelry market that was hit by one of the blasts. Jain said he was "disgusted with politicians who promise security, but do nothing after the media cameras are gone."

The bombings marked the worst attack in India since the 2008 siege, which killed 166 people over three days.

After that attack, the government expanded police recruiting and training, bought high-tech equipment and updated its ancient police arsenal. It established a National Investigation Agency to probe attacks and set up commando bases across the country — including one in Mumbai — so rapid reaction forces could swiftly arrive at the scene of an attack.

Chidambaram said state and national intelligence agencies were working far more closely than in the past and intelligence collection was far more extensive. The 31-month gap between attacks in Mumbai underscored the large number of foiled threats, he said.

However, the law enforcement system in the country was so badly degraded that even these changes have done little to increase safety, said Ajai Sahni, executive director of the Institute for Conflict Management.

He called the NIA "a tiny little organization," that is badly underresourced. "It is not the FBI."

While the police have improved, arriving on the scene of the blasts within minutes Wednesday, their training, forensic and investigative capabilities remain horribly deficient, leaving them powerless to uncover terror plots before they are carried out, he said.

"We thought we were safe," said Anita Ramaswami, a 33-year-old accountant. "But things still are the same and people in Mumbai continue to feel vulnerable.

The sheer number of targets across a country of 1.2 billion, makes it nearly impossible to protect, officials said.

"It's very difficult to stop every single terror attack," said Rahul Gandhi, a senior leader of the ruling Congress Party.


A police officer stands guard at the site of an explosion in the Zaveri Bazaar, in south Mumbai, after a string of three bomb attacks. ((Danish Siddiqui/Reuters))

At Chhatrapati Shivaji Terminus, the Mumbai train station where 52 people were gunned down in the 2008 attacks, armed railway police — some of them behind sandbagged barricades — struggled Thursday to monitor the crush of passengers.

An estimated 3.75 million commuters on more than 1,600 trains pass through India's busiest train station every day.

"The crowds are so dense during peak hours it would be impossible to keep a check, even with the most stringent security," said station manager D.K. Gupta.

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.

Investigators protect evidence from driving rain

At the scene of the bombings, investigators struggled to preserve evidence with plastic sheets as a driving rain washed away the bloodstains.

One bomb had been placed on a bus shelter, another was hidden under some garbage on the road, while the third was stashed under an umbrella, officials said. All were improvised explosive devices made of ammonium nitrate with electronic detonators, authorities said.

"The IEDs were not crude and showed some amount of sophistication and training," said R.K. Singh, India's home secretary.

Investigators were viewing closed circuit television footage and speaking to wounded witnesses to try to put together a picture of what happened at each location, Rakesh Maria, the head of Mumbai's Anti-Terror Squad, told reporters.

Rakesh Mehta, an accountant who travels every day through the warren of narrow lanes and tiny goldsmith workshops in the jewelry market, said he was badly shaken.

"In these uncertain times, I find myself stopping at any temple that I pass," he said.

Indian officials refused to speculate on who might be behind the attack.

Opera House, Mumbai

"We are not pointing a finger at this stage," Chidambaram said. "We have to look at every possible hostile group and find out whether they are behind the blast."

A former top intelligence official, who spoke on condition of anonymity because of the sensitivity of the investigation, said the attack had the hallmarks of the Indian Mujahideen, an Islamic militant group linked to Pakistan's Lashkar-e-Taiba that has claimed past attacks that used similar explosives.

Local police arrested two members of the group in recent days and there was speculation the blasts could have been retaliation.

Indian officials have accused Pakistan's powerful spy agency of helping to co-ordinate and fund earlier attacks, including the 2008 Mumbai attack. Peace talks between the countries were suspended after that attack and resumed only recently.

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.

The Hindu nationalist opposition labelled Pakistan the hotbed of terror in the region, called for its spy agency to be declared a terror outfit and criticized the Indian government for not dealing more sternly with Islamabad.

"The government of India must shed its ambivalent attitude to terrorism. The total policy of India toward terrorism should be of zero tolerance," said L.K. Advani, a senior leader of the opposition Bharatiya Janata Party. "Our message to Pakistan should be that you must dismantle the infrastructure for terrorism that you have created."