Indian officials say it is too early to accuse any particular group of carrying out the three consecutive bomb blasts that ripped through the country's financial capital, Mumbai, on Wednesday, killing at least 17 people and wounding 133 others.

Indian Home Minister P. Chidambaram said Thursday police are investigating every possible “hostile” group, and that Indian intelligence had no warning before the bombings. No one has claimed responsibility.

The two explosions in south Mumbai and one in the central part of the city occurred within 20 minutes of each other during the evening rush-hour.

Earlier reports said the blasts killed 21 people, but the Home Ministry revised the death toll Thursday and said the number could still change.

Home Minister Chidambaram said the government believes the blasts were a “coordinated terror attack” because of their close timing. He visited all three blast sites Thursday, as well as a local hospital to meet with those wounded in the attacks.

The Home Ministry has ordered security heightened across the country.

Police believe the blasts were caused by improvised explosive devices. They say that in two of the blasts, the bombs were attached to motor vehicles, while the third occurred on top of an electrical metering box above a billboard.

Indian Prime Minister Manmohan Singh condemned the attacks, as did the Pakistani government.

U.S. President Barack Obama called the bombings “outrageous attacks” and pledged support to India's efforts to bring those responsible to justice.

U.S. Secretary of State Hillary Clinton offered condolences to the Indian people and government. Clinton said she will go ahead with her trip to India next week as planned and that she believes “it is more important than ever that we stand with India.”

Wednesday's attack is the worst to hit Mumbai since Pakistan-based militants laid siege to the city in 2008, killing 166 people.

It happened just two days after the fifth anniversary of a series of train bombings in Mumbai that killed more than 180 people. Indian authorities blamed that attack on Pakistani militants.