MANILA -- The Philippine National Police (PNP) on Monday ordered an investigation into the ambush of 5 agents of the Philippine Drug Enforcement Agency (PDEA) in Lanao del Sur last Friday.

PNP Chief Director-General Oscar Albayalde said they have in custody all the policemen who were in the agents' convoy during the ambush as they piece together what happened.

"They were all relieved from their posts pending investigation although hindi sa atin masabi ng regional director doon kung talagang iniwan intentionally 'yung mga PDEA agents," he said.

Albayalde also said they are already looking at one group related to illegal drugs that may be behind the ambush, although their information has yet to be validated.

Three of the agents killed in the ambush were fresh graduates of the PDEA academy. They were on board a van on their way to Marawi City when there were waylaid by the gunmen. --report by Raffy Santos, ABS-CBN News