Metropolitan Manila police director Nicanor Bartolome said investigators had recovered parts of a remotely detonated improvised bomb that resembles similar devices used by militants in the volatile southern Philippines, where minority Muslims have been fighting for self-rule for decades.
President Benigno Aquino III tried to calm residents in a televised news conference shortly after Tuesday's blast along a major thoroughfare in the Makati financial district, promising his government will not allow fear to settle in.
He and other officials have not named any specific group, but suggested the possible involvement of al Qaeda-linked Abu Sayyaf militants. They were also blamed for a bus bombing in 2005 that killed four near the site of Tuesday's blast. A year earlier, the Abu Sayyaf was blamed for the country's worst terrorist bombing that killed 116 people in a burning ferry in Manila Bay.
Aquino said he directed his national security adviser to reassess threats that reportedly came from terrorist groups last year and that prompted several countries, including Australia, Britain and the U.S., to put in place travel warnings. At the time, the government had not found the threats credible and the warnings infuriated Aquino.
Investigators were also looking into the background of the wounded and the dead, police Chief Superintendent Jose Arnel delos Santos said. His statement suggested that one of the passengers may have been carrying the explosive when it went off prematurely.
Police forces were put on alert and security in possible targets, including airports, hotels, bus terminals and public places, was strengthened. Bartolome also said that marshals will be boarding some of the 7,000 buses that ply the streets of Manila every day.
The Abu Sayyaf is on the U.S. list of terrorist organizations. One of its bombs killed two U.S. soldiers on southern Jolo Island in September 2009.
Although they have been considerably weakened by battle casualties and surrenders, the militants still number about 340 and remain a serious threat, according to the military. Hundreds of U.S. troops have been training Filipino soldiers in the south since 2002.