Three blasts ripped through Mumbai during rush hour Wednesday evening, killing at least 21 people and injuring more than 110, Indian officials said. The explosions, which officials described as a terrorist attack, targeted crowded areas of the city, India’s financial capital.

“The blasts occurred at about 6:45 p.m., within minutes of one other. Therefore, we infer that this was a coordinated attack by terrorists,” said P. Chidambaram, India’s home minister, speaking in New Delhi.

In Washington, President Obama issued a statement denouncing the bombings as “outrageous” and pledging U.S. support for India. No group has asserted responsibility for the blasts, which struck the busy middle-class neighborhood of Dadar, the jewelry street of Zaveri Bazaar and the upscale Opera House district.

Zaveri Bazaar was the site of bombings in 2003 and 1993, as well. “This jewelry market is a vulnerable area, with very crowded, narrow alleys. It is not easy to monitor people’s suspicious movements or check who is carrying what,” said Sanjay Nirupam, a a federal lawmaker from Mumbai.

Mumbai’s police chief, Arup Patnaik, told reporters that an improvised explosive device had been hidden in an umbrella in Zaveri Bazaar, adding, “We are finding out what kind of explosive was used.”

Terrorists have targeted the city 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.

“I think we let our guard down when things returned to normal after the last attacks,” said Neel Pillai, a banker who lives a few miles from the bombing sites. “We became too complacent.”

The major concern for U.S. diplomats, observers said, is how the blasts could affect India-Pakistan ties, which had only recently begun to recover from the 2008 siege.

One obvious suspect in the attacks Wednesday is Lashkar-i-Taiba, a violent Islamist group that Pakistan once sponsored as a proxy army against India and that is suspected of being behind the 2008 attacks, said former CIA officer Bruce Riedel, a senior fellow at the Brookings Institution. Another strong possibility, he said, would be an Indian-based Muslim extremist group, such as the Indian Mujahideen, which has been linked in the past to smaller-scale bombings.

“This one’s fairly hard to place,” Riedel said. “It has elements of sophistication, with multiple targets and coordinated attacks, which could suggest Lashkar, but it doesn’t have the large-scale ambition you saw in the 2008 attacks.”

Pakistani President Asif Ali Zardari and Prime Minister Yousaf Raza Gillani condemned Wednesday’s blasts in a statement. But bilateral tensions will escalate quickly if investigators discover a Pakistani connection, Riedel said, especially in light of the recent U.S. raid in Pakistan to kill al-Qaeda leader Osama bin Laden.

“The bin Laden kill puts an awful lot of pressure on India to do something similar,” he said.

The raid also makes it more difficult for the United States to counsel restraint, Riedel said. “Traditionally, we’ve told India to exercise restraint, take it on the chin. If Pakistan was involved, it’s difficult to see how we can make that case in light of our own action on bin Laden.”

At a news conference Wednesday, Secretary of State Hillary Rodham Clinton talked of the United States’ and India’s “shared struggle against terrorism.” Alluding to her scheduled trip to the subcontinent next week, she added, “I believe it’s more important than ever that we stand with India.”

Wan reported from Washington.