No terror group has claimed responsibility for the triple bombings that killed 17 people in India's financial capital, and investigators had no immediate suspects in the attacks that came without warning, the country's top security official said Thursday.

Angry residents of Mumbai blamed the government for an apparent intelligence breakdown that left the city vulnerable, but Home Minister Palaniappan Chidambaram said the latest attack did not represent a failure of the country's anti-terror network.

"Whoever has perpetrated this attack has worked in a very, very clandestine manner," Chidambaram said at a news conference after an emergency security meeting.

The bombings shook three separate neighborhoods within minutes during Wednesday's busy evening rush and were the country's worst terror strike since the 2008 siege of Mumbai that killed 166 people. After that, India enacted sweeping security reforms and has avoided another major attack in the nearly three years since.

That did little to reassure residents who questioned how the attack could happen despite massive security measures taken in recent years.

"After the 2008 blast and all the media hype (about safety), 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."

Top Indian officials said the government may never be able to guarantee a terror-free nation in a region so plagued by extremist groups.