A day after a rush-hour triple bombing killed 18 people in Mumbai, many angry Indians questioned whether their government had learned any lessons since the last deadly attack in the city, in 2008.

“Why is Mumbai being targeted again and again? Trains, bus stops, markets, hotels — nothing is safe anymore. After the last attack, the government promised us that this will never happen again,” Swati Kamat, a 30-year-old corporate executive, said in a telephone interview. “I feel sad, but I feel much more anger than sadness. I am angry at the government, at the police, at our system.”

The anger was palpable on the streets, in offices and in television debates Thursday. Prime Minister Manmohan Singh echoed the mood when he arrived in Mumbai, India’s financial capital.

“I understand the shock and outrage of the people of Mumbai. I share their pain, anguish and anger,” Singh said. He promised that his government would “relentlessly pursue the perpetrators.”

But Thursday, police officials said no conclusive leads had emerged from a preliminary probe into the explosions.

“Nothing conclusive can be said with confidence about which militant group was behind these attacks. But it is not a blind investigation, either,” said U.K. Bansal, a senior internal security official in the Home Ministry in New Delhi.

No group has asserted responsibility for the blasts, which initial reports said had killed 21 people.

The bombs exploded Wednesday evening in marketplaces and other crowded areas of Mumbai, including a street lined with jewelry shops. At least 133 people were injured, and 23 of them were in serious condition.

Forensic teams combed through the rain-drenched blast sites looking for evidence and examined the grainy images captured on surveillance cameras installed on the streets. But with monsoon rains falling in Mumbai at the time of the attack, the faces of many of the people captured on camera were obscured by umbrellas. The rain also delayed analysis of potential evidence taken from the areas.

Officials described the bombs as improvised explosive devices, made with ammonium nitrate and triggered by a timer mechanism and said they were concealed in trash under a cart, on top of a billboard at a bus stop and under an umbrella near a motorcycle.

Police also discovered remnants of wires and a battery on one of the bodies found near a blast site. Bansal told reporters that the possibility of the involvement of a suicide bomber “is not ruled out.”

But, unlike in the past, New Delhi officials did not rush to blame neighboring Pakistan or any Islamist group.

“We are not pointing our fingers, at this stage, at this group or that group,” P. Chidambaram, the home minister, said at a news conference in Mumbai on Thursday morning. “All angles will be examined without any predetermination. All groups hostile to India are on the radar.”

Ongoing investigations into a few recent attacks previously attributed to Muslim organizations have revealed the involvement of radical Hindu groups.

One senior intelligence official in New Delhi said that the explosions Wednesday bore the signature of the Indian Mujahideen, an indigenous group of young, radicalized Muslims who say India treats Muslims unfairly. The group has been accused in several bombings in Indian cities in 2007 and 2008.

Other officials, however, said it would be premature to name that group as a suspect.

On Thursday, Mumbai was under a security alert, and commuters were required to pass through police checkpoints in various parts of the city. But schools and offices opened for business in the morning.

Terrorists have targeted Mumbai multiple times since 2000, including a siege in November 2008 that killed 166 people. A lone surviving gunman, of Pakistani origin, was convicted in that case on charges of terrorism, criminal conspiracy and waging war against the Indian state.

The jewelry market targeted in Wednesday’s attack, Zaveri Bazaar, also was a site of bombings in 2003 and 1993.

After the siege in 2008, New Delhi suspended talks with Pakistan. But the two nuclear-armed rivals resumed dialogue this year, mostly because of prodding by U.S. officials. The next round of talks is set to take place in two weeks.

Speaking by telephone from Saifee Hospital in Mumbai, 20-year-old migrant Dinesh Kumar said he lost a leg in the blast.

“I left my village and came to Mumbai just a month ago to earn a livelihood. I don’t know this city at all. I had just started working as a laborer in a small shop,” Kumar said. “Some strangers carried my bleeding body to the hospital. I don’t know where to go, what I will do now.”