Manila, Philippines ? A bomb that injured 30 people on a passenger ferry appears to have been designed to start a fire rather than cause a large explosion, a Philippine police spokesman said Monday. A military official said the investigation was focusing on Abu Sayyaf, an al-Qaida-linked group active in the region.

The ferry was docked at the wharf at Lamitan and preparing to depart with more than 300 passengers when the bomb, hidden in a cardboard box filled with old clothes, went off.

“It was found that low explosive substance was used here, unlike in previous bombings,” said Leopoldo Bataoil, a spokesman for the Philippine National Police. “If it’s a low explosive, the effect would be a burning effect that could cause fire.”

He said it was possible the perpetrators’ aim was “to instill fear and panic.”

The south is the homeland of the country’s Muslim minority and a decades-old Islamic separatist insurgency.