DIGOS CITY, Philippines -- Another local bus company has been threatened by the al-Khobar gang, an extortion group blamed for the series of bomb attacks on bus firms operating in southern and central Mindanao regions, the police said Thursday.

But Senior Superintendent Cesario Darantinao, Davao del Sur police chief, declined to name the bus company "to avoid undue alarm among the riding public."

Darantinao said officials of the bus company sought assistance from the police after receiving threatening calls, allegedly from the al-Khobar, a few days after the bombing here of a Davao Metro Shuttle bus on September 1 that killed six persons and wounded more than 30 others.

Another attack on a Metro Shuttle bus in July killed two persons.

The gang, led by Salahuddin Hassan and Abdul Malik Salih, has been blamed for attacks against units of the Weena Bus Co., Yellow Bus Co. and Metro Shuttle, in which nearly two dozen people have died since 2006.

Authorities said the al-Khobar, which is based in Central Mindanao, had demanded protection money from the transportation firms.

Metro Shuttle, owned by the family of Tagum City Mayor Rey Uy, has repeatedly rejected the al-Khobar’s demand for about P50,000 per month.

In both attacks on Metro Shuttle buses, the gang used a woman as courier, Darantinao said.

Darantinao said police are not taking chances and have tightened security to prevent a repeat of the attacks.

But he said bus companies also need to cooperate because the police can not ensure the safety of passengers at all times.

Darantinao said bus companies should order their drivers to stop picking up passengers from the roadside between points of destination.

"We have already deployed police officers in terminals to check on incoming passengers and buses but we do not have personnel to check on passengers, who board the buses from outside terminals," he said.

Darantinao pointed out that in recent attacks, the suspected bombers had boarded the buses in between terminals.

Meanwhile, Darantinao said charges had already been filed against al-Khobar gang members responsible for the September 1 and July 24 attacks here.

But none of those charged, including suspected courier Junaira Mimbida, have been arrested so far.

Darantinao urged the people to give information that could lead to the arrest of the al-Khobar members, for whom a total bounty of P1.7 million has been raised.