ZAMBOANGA CITY -- Police and military authorities have tightened security measures after separate explosions rocked this city early Friday.

Initial investigation showed that the first bomb exploded shortly after 4 a.m. in Sevilla Street, few meters away from the city’s commercial district.

It was followed by another explosion at 4:15 a.m. in Plaza Pershing, few meters away from the site of the first blast and from City Hall.

No one was either killed or injured in the two explosions.

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Senior Superintendent Edwin de Ocampo, Zamboanga City Police Office (ZCPO) director, said investigation continues to unmask the suspects behind the attacks.

De Ocampo said all possible motives are being explored by the investigators.

Public Utility Jeepney (PUJ) drivers utilized Sevilla Street as their terminal, while Plaza Pershing is frequented by people at day time.

De Ocampo said the post-blast investigators have found traces of ammonium nitrate at the two blast sites.

The incident took place just two weeks after a bomb scare hit the City Hall, forcing the evacuation of all employees and people transacting business.

Earlier reports said a City Hall executive assistant received the bomb threat through a text messages around 9 a.m. of January 15, saying bombs were planted inside the historic building and were set to go off in an hour.

Mayor Celso Lobregat, when informed of the text message, hinted it to be a hoax but did not take the threat lightly as he immediately informed the top police and military officials of the report.

"It is always better to be safe than sorry," Lobregat said.

No bomb, however, was found by the police Explosive Ordnance Demolition (EOD) personnel who inspected the entire City Hall building following the alert.

Zamboanga City has been marred by bombings, ambushes, and several other attacks in the past months, prompting local officials to join forces to protect their boundaries from lawless elements.

Police Action Centers have been put up in the city to ensure the protection and safety of inter-provincial passenger buses and the residents in the area. (Bong Garcia/Sunnex)