MANILA - Police are investigating claims that drug-laced drinks were distributed Saturday night at a concert in Pasay City where five people died.

The fatalities were identified as Ariel Leal, 33; Lance Garcia, 26; Bianca Fontejon, 18; and a male American national, 33. The fifth victim, 18-year-old Ken Migawa, was earlier reported to be in critical condition, but also died Sunday night.

The victims were found unconscious at different parts of the SM Mall of Asia Open Concert Grounds during the concert organized by toothpaste brand Close-up.

Chief Insp. Rolando Baula, Pasay police investigation chief, said an autopsy showed Fontejon and Kigawa suffered massive heart attacks.

Baula told radio dzMM that they are still awaiting results of the toxicology examination to determine what caused the heart attack.

The family of Leal declined to have his remains undergo autopsy, the police official added.

Police on Monday were also set to inform the U.S. Embassy of the death of the American national, whose remains were still at the Manila Doctors Hospital.

A report on ABS-CBN's Umagang Kay Ganda, meanwhile, said police are seeking confirmation if the deaths were caused by drug-laced drinks.

Several netizens speculated that a party drug called "Green Amore" was given to the victims, according to another report on radio dzMM.

Green Amore is a capsule supposedly containing a lethal mix of ecstacy and shabu. Also known as "fly high" and "Superman", the drug can allegedly cause side effects of dehydration, fever and heart attack.

A man who passed by SM Mall of Asia on Saturday also told ABS-CBN's Bianca Dava that the concert venue smelled of drug fumes. He also claimed spotting several supposed minors at the concert.

"Sobrang wild nung kagabi na iyun. Sobrang dami ng tao tapos makikita mo talaga na maraming bata," said the man who requested anonymity. "Noong andoon na kami sa area, maaamoy mo naman talaga na parang amoy weeds siya, amoy marijuana iyung area."

Police are still investigating the incident. They have not monitored any other reported injuries so far during the concert billed as "summer's biggest rave party."

Close-up and its media partner, Eventscape, said in a joint statement that stringent measures and precautions were put in place to ensure the safety of all concert attendees.

They added that strict protocols were followed in providing immediate medical assistance to the victims and in rushing them to the nearest hospital.

They also committed their support to the families of the victims while ensuring full cooperation with the police investigation.

SM Supermalls, likewise, expressed its sympathies. Company president Annie Garcia said SM is assisting the event organizers and authorities in their investigation. -- With reports from Bianca Dava, ABS-CBN News; Fred Cipres, dzMM