Police and soldiers in the capital went on high alert, but Interior Secretary Ronaldo Puno sought to downplay the possible involvement of Muslim extremists, saying the investigation is "pointing away from terrorist attack and more of a directed assault on a certain individual."
Rep. Wahab Akbar's chief of staff said the congressman died of wounds in a hospital, and metropolitan Manila police chief Geary Barias said a lawmaker's driver and a congressional staff person also were killed. Seven others were injured, including two congresswomen.
"There were threats on the life of Akbar," Puno told reporters. "The indications are that that was the case both in terms of location of the bomb and the manner it was set off."
Akbar, a former governor of southern Basilan province, had been targeted by the Abu Sayyaf Muslim extremist group in the past for launching offensives against them.
But he also had political opponents, including ones who ran against one of his wives who succeeded him as governor of Basilan. Political rivalries in the southern Philippines are often solved with readily available weapons, and assassinations of politicians are common.
National police chief Avelino Razon said the blast was caused by a bomb.
A number of cars were damaged outside the southern entrance to the building, where the blast hit after 8 p.m. as the House ended its session and lawmakers and their staff were being picked up by their drivers.
Razon said a destroyed motorcycle was found and experts were conducting chemical tests to find out if it was used to carry the bomb.
Investigators suspect the bomb may have been placed on one of two parked motorcycles and then remotely detonated as Akbar approached his car, fatally wounding him and ripping the motorcycles apart, Barias said.
"I felt the blast although I was on the other side of the building. The ceiling of the canopy near the south wing entrance came down," Rep. Teodoro Casino told The Associated Press.
Police cordoned off the massive House complex in suburban Quezon City shortly after the explosion.
Puno said authorities were treating the blast as "as a violent crime intended on a particular individual rather than any institution."
President Gloria Macapagal Arroyo said while police investigate and bolster security, "we're making a call against rumors, accusations that create confusion, fear and conflict."
"If this is terrorist action or work of an anarchist I'm sure it was deliberately done to cow us," House Speaker Jose de Venecia told reporters at the site.
The blast occurred amid heightened political tensions in the country. Arroyo is facing a third impeachment complaint in as many years.
"We cannot rule out anything until the investigation is completed," de Venecia said. "There are many threats to us personally and officially. We will have to decide whether we have to augment security."
"We are not afraid (and) we want the situation to normalize as quickly as possible," he added.
The Philippine capital has been jittery since last month, when an explosion damaged a shopping mall in the financial district, killing 11 people and injuring more than 100. A preliminary police report said it was an accident, although the owners of the mall disputed the finding.
Muslim militants also have bombed targets in the capital in the past.
The slain congressman, Akbar, was once a member of the Moro National Liberation Front, a Muslim rebel group that dropped its secessionist goal and signed a peace accord with the government in September 1996.
Some security officials have suspected that Akbar knew the leaders of the Abu Sayyaf, a Muslim radical group that has its roots on Basilan island. But they said he later had a falling out with Abu Sayyaf commanders and started fighting them.
Representatives of the Israeli Defence Ministry responded to recent reports about the possible delivery of S-300 SAM systems from Russia to Syria. Israeli Defence Minister Avigdor Lieberman said that Israel would destroy those systems
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