Suspicion fell on Abu Sayyaf because of similarities to an Oct. 2 blast that killed four, including an American Green Beret. That explosion was blamed on the Muslim extremists notorious for kidnappings and murders. TNT was apparently used in both attacks.
Thursday's bombings, 30 minutes apart, came less than a week after three bombs exploded on the Indonesian island of Bali, killing 183 people.
There was no claim of responsibility for the attacks in Zamboanga, but military spokesman Lt. Col. Danilo Servando said suspicion fell on an Abu Sayyaf faction headed by Khaddafy Janjalani, one of five leaders of the group indicted by Washington for a mass kidnapping last year that left 18 hostages dead, including two Americans.
Government officials have said Abu Sayyaf may have links with Jemaah Islamiyah, another Islamic militant group in Southeast Asia that allegedly has ties with the al Qaeda terror network.
On Thursday, Indonesian police named Abu Bakar Bashir, the spiritual leader of Jemaah Islamiyah, a suspect in a series of church bombings across Indonesia on Christmas Eve 2000. The group has been linked by reports to the Bali bombings last weekend.
Days before the Oct. 2 bombing, Abu Sayyef had threatened attacks in retaliation for a military offensive against it.
In linking Abu Sayyaf to al Qaeda, Philippine officials have cited alleged attempts by Osama bin Laden's lieutenants to provide the guerrillas with training on explosives and weapons handling in past years.
Thursday's attack turned the usual early afternoon bustle of Zamboanga's downtown area into a gory scene of debris, blood and chaos. Panicked shoppers scattered as police cars and pickup trucks were mustered into ambulances to ferry wounded people to hospitals that were quickly overwhelmed.
Nervous police bomb squads blew up five suspicious packages, but further examination found they contained no explosives.
President Gloria Macapagal Arroyo condemned the latest terrorist strike to hit her impoverished country, a key supporter in the U.S. war on terrorism.
Some 260 American troops are in Zamboanga, a predominantly Christian city of 600,000 in the violence-wracked southern Philippines, after a six-month counterterrorism training exercise to help local troops fight the Abu Sayyaf.
No foreigners were believed to have been injured in Thursday's bombings.
National Security Adviser Roilo Golez refused to speculate on possible links to the deadly bombings on Bali, saying officials were looking at the Zamboanga attack as "a local concern."
"There is no need to declare a state of emergency and the public has nothing to worry about," Golez said.
Police said 10 people were brought in for questioning, including two foreigners.
The first blast Thursday happened at 11:30 a.m. at the Shop-o-Rama department store. It was followed 30 minutes later by a second explosion at the adjacent Shoppers Central store. Police Chief Mario Yanga said the bombs were placed at counters where shoppers leave packages as they enter the stores.
"The ground shook and pandemonium broke out. People bathed in blood were all screaming and running away from the smoke," said Ofelia Fernandez.
Television footage showed victims on stretchers being hauled out of the bombed buildings' lobbies. A bloodied man, with most of his shirt and pants ripped away by a blast, staggered out, helped by a policeman.
Firefighters poured water onto the wreckage as medical workers rushed people on gurneys to waiting ambulances.
A truckload of soldiers arrived to secure the area, and a pair of MG-520 attack helicopters hovered overhead while armored personnel carriers patrolled the ground. Checkpoints quickly were thrown up.
All shops in the city's commercial center closed, and police blocked off streets with cars and yellow tape. Trapped employees were later led out one by one. Many schools around the city sent students home.
Most public buildings in the Philippines' metropolitan areas post armed guards at entrances, sometimes with metal detectors, who are supposed to search packages. But Yanga said they often become complacent.
"These malls are always being lectured, but their security remains lax," he said.
Elsewhere, Philippine troops captured a sprawling Muslim separatist guerrilla camp as they continued offensives against rebel groups on two fronts in the southern Philippines, military officials said Thursday.
At least 46 guerrillas of the Moro Islamic Liberation Front have been killed in six days of artillery assaults and air strikes in Sultan Gumander town in Lanao del Sur province, the officials said.
The 38- to 50-acre rebel camp there, which had 40 bunkers, trenches, foxholes and dugout guard posts, was destroyed, a military statement said without saying when the guerrilla site fell. Soldiers recovered rebel documents and combat packs.
MILF spokesman Eid Kabalu said just four guerrillas were killed and none of its camps had been taken.
Meanwhile, troops were continuing to search for about 100 Abu Sayyaf guerrillas who nearly wiped out a marine platoon, killing 11 soldiers and wounding 26 others on Jolo island Saturday, officials said.