MANILA, Philippines?(UPDATE 10) At least four people were killed and 14 others injured when an ?big bomb? tore through a packed passenger bus travelling along Edsa on Tuesday, officials said.

The blast was so powerful that it created a hole on the Newman Gold Liner bus' floor and body and damaged its side windows, Metro Manila police chief Director Nicanor Bartolome said.

Police were considering all motives for the attack, including terrorism and destabilization, he said.

?We cannot make any conclusion yet. Our special investigation task force is still working, processing evidence gathered on the site,? Bartolome added.

The explosion occurred around 2 p.m. while the bus was approaching the loading bay near MRT Buendia railway station on Edsa's northbound lane, said Ed Garcia, radio operator at the Metropolitan Manila Development Authority (MMDA) metrobase.

Shattered windshield, windows

Bartolome said the sixth row on the right side of the bus bore the brunt of the explosion.

A photo provided by the MMDA showed a shattered windshield and side windows of the ill-fated bus (plate number TXJ 710).

Police investigators were seen scouring the scene for evidence, with blood drying on the pavement.

The driver, who survived the attack unscathed, said at least two men who acted strangely quickly got off the bus just moments before the explosion.

"They were moving from one seat to another although we had no idea there was an explosive aboard," the driver, Maximo Peligro, told reporters.

"It could not have been mechanical... it was very strong."

Jam-packed

He said about 30 people were on board the bus when the blast hit.

Bartolome said they have yet to determine the type of explosive used.

?Our investigation continues, we have yet to gather all the debris and reconstruct the fragments to categorically determine what type of explosive material was used,? said Bartolome.

"We have to complete the investigation. We need to know who among the victims could be interviewed to shed light on the incident. We need to be certain,? Bartolome said.

?We have to determine all the pieces of evidence recovered from the scene. The footage taken by the closed-circuit television (CCTV) camera of the Metropolitan Manila Development Authority would be a big help,? he added.

A bomb squad unit and a contingent of the military's National Capital Region command were deployed to the area.

Full alert

The entire 136,000-strong Philippine National Police force was placed on full alert status nationwide effective 5 p.m. Tuesday. All seaports, airports and the railway transport system were also on a full alert.

The attack occurred two months after the US and several other Western governments warned a terrorist attack in Metro Manila was imminent.

Makati Mayor Junjun Binay, who arrived with his father, Vice President Jejomar Binay, said the explosion was so powerful that it punched a hole in a nearby concrete fence.

The younger Binay said that when he arrived at the site 10 minutes after the explosion, he saw blood and severed body parts inside the bus. He said he also saw a severely injured man through the big hole created by the blast.

Two of the victims, who had not been identified as of this posting, died on the spot. Two other passengers died in the hospital, police said.

Some of the victims were brought to Ospital ng Makati while the others were rushed to St. Luke?s Medical Center at the Bonifacio Global City.

Amputated limb

One passenger was severely injured that a limb had to be amputated, the National Disaster Risk Reduction and Management Council said.

"Some of the passengers who were not hurt already went home," Bartolome said.

The driver was in police custody for investigation, said Chief Inspector Jenny Tecson of the Southern Police District. The bus conductor, identified as Michael Haralde, is reportedly wounded.

In an interview with reporters at the Makati City Police Station, Peligro said he smelled gunpowder right after the blast.

?If it was a normal explosion, there would be no smell of gunpowder," the bus driver said, insisting that mechanical failure could not have been the cause of the blast because the bus had been well-maintained.

Peligro also dismissed the possibility that the attack on the bus was done by enemies of the bus company's owner.

?We have devotions before we leave the terminal,? he said.

Peligro, who was unharmed, also said that the bus? windshield was not damaged by the explosion. He said he kicked it to enable him and the other surviving passengers to get off the vehicle.

According to Haralde, the bus came from Baclaran in Parañaque City and was bound for SM mall in Fairview, Quezon City.

Haralde, who sustained injuries on both legs, said they had 20 passengers when they left Baclaran, and it became almost full when they stopped at a loading bay in Evangelista area in Pasay City. He said almost half of the passengers disembarked at Edsa Ayala.

The driver and the conductor said the explosion happened before they reached Buendia at the corner of Edsa.

Other victims in the blast are identified as Mabel Gozon, Mary Grace Borondian, Antonio Lino, and Jeffrey Victoriano. They were brought to the Ospital ng Makati for treatment.

Ten others were rushed to St. Luke's Hospital in Global City, Taguig.

Tecson said one of those victims died while being treated in the hospital while one unidentified female and a certain Jhohanson Reyes died on the spot.

Mariano Salustiano, who was brought to the Ospital ng Makati, was also declared dead later on.

Terror alerts

In November, the US, Australian, British, Canadian, French and New Zealand governments updated their travel advisories to the Philippines to warn that terrorist attacks were likely to occur soon.

Some of the warnings specifically mentioned that an attack would occur in Metro Manila.

President Benigno Aquino strongly criticized the advisories, saying they were not based on firm intelligence, prompting some of the governments to downgrade the threat assessment.

However, the US maintained its warning, which could still be viewed on its Manila's embassy's website on Tuesday.

Security threats

The Philippines has a range of security threats, most prominently the Abu Sayyaf Muslim militant group that operates mainly in the south of the country, and communist rebels who have been waging a rebellion for 42 years.

The Abu Sayyaf bombed a passenger ferry on Manila Bay in 2004 that killed over 100 people in the country's worst terrorist attack, according to authorities.

The Abu Sayyaf was also blamed for placing a bomb attack on a passenger bus near Makati in 2005 that left three people dead.

Armed gangs and other extortionists are also known to attack buses in the southern Philippines frequently.

Nine passengers were killed in one such attack in October last year that was blamed on former Muslim separatist rebels seeking to extort money from a local bus company.

With reports from Tina Santos, Philippine Daily Inquirer and The Associated Press